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Clara Barton became known as "The Angel of the Battlefield" during the American Civil War. Born in Oxford, Massachusetts in 1821, Clara Barton's interest in helping soldiers on the battlefield began when she was told army stories from her father. Another event that influenced her decision to help soldiers was an accident her brother had. His injuries were cared for by Barton for 2 years. At the time, she was only Il years old. Barton began teaching school at the age of 15. She taught for 18 years before she moved to Washington, D.C. in 1854. The civil war broke out 6 years later. Immediately, Barton started war service by helping the soldiers with their needs. At the battle of Bull Run, Clara Barton received permission from the government to take care of the sick and hurt. Barton did this with great empathy and kindness. She acknowledged each soldier as a person. Her endurance and courage on the battlefield were admired by many. When the war ended in 1865, she used 4 years of her life to assist the government in searching for soldiers who were missing during the war. The search for missing soldiers and years of hard work made her feeble physically. In 1869, her doctors recommended a trip to Europe for a rest. While she was on vacation, she became involved with the International Red Cross, an organization set up by the Geneva Convention in 1864. Clara Barton realized that the Red Cross would be a big help to the United States. After she returned to the United States, she worked very hard to create an American Red Cross. She talked to government leaders and let American people know about the Red Cross. In 1881, the National Society of the Red Cross was finally established with its headquarters in Washington, D.C. Clara Barton managed its activities for 23 years. Barton never let her age stop her from helping people. At the age of 79, she helped flood victims in Galveston, Texas. Barton finally resigned from the Red Cross in 1904. She was 92 years old and had truly earned her title "The Angel of the Battlefield". Which of the sentences best expresses the essential information in the highlighted sentence? Incorrect answer choices change the meaning in important ways or leave out essential information

Xem chi tiết 1.2 K lượt xem 5 năm trước

Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blanks from 31 to 35.   Alligators have always been the most dangerous predators of the Florida Everglades, (31) .................. their long-held position is now being challenged by a growing population of Burmese pythons.   The Burmese python, a giant snake measuring up to 20 feet long and weighing up to 200 pounds, is very popular among exotic pet owners. However, as the (32) .................. snakes grow, they become more difficult to cafe and handle. To avoid dealing with the snakes, the owners irresponsibly release them into the wild. (33) .................. the Everglades is quite similar to the python’s native environment of Southeast Asia, they survive and prosper without difficulty.   The issue with the Burmese pythons is their choice of meals. They eat alligators and endangered birds, which creates a strain on an already (34) .................. ecosystem. Recently a 13-foot long Burmese python was found with a 5-foot alligator bursting from its stomach. They python died trying to eat the alligator but a larger one would have easily won the struggle. With over 30,000 Burmese pythons now living in the Everglades, a solution to stop this invasive species is necessary. If the pythons are (35).................. unchecked, there is a strong possibility they will wipe out a variety of species necessary for the functioning of the Everglades ecosystem. (Source: * Master TOEFL Junior*, Hahn (2015) Điền vào số 31

Xem chi tiết 539 lượt xem 5 năm trước

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 30 to 34. An air pollutant is defined as a compound added directly or indirectly by humans to the atmosphere in such quantities as to affect humans, animals, vegetation, or materials adversely. Air pollution requires a very flexible definition that permits continuous change. When the first air pollution laws were established in England in the fourteenth century, air pollutants were limited to compounds that could be seen or smelled - a far cry from the extensive list of harmful substances known today. As technology has developed and knowledge of the health aspects of various chemicals has increased, the list of air pollutants has lengthened. In the future, even water vapor might be considered an air pollutant under certain conditions. Many of the more important air pollutants, such as sulfur oxides, carbon monoxide, and nitrogen oxides, are found in nature. As the Earth developed, the concentration of these pollutants was altered by various chemical reactions; they became components in biogeochemical cycles. These serve as an air purification scheme by allowing the compounds to move from the air to the water or soil. On a global basis, nature's output of these compounds dwarfs that resulting from human activities. However, human production usually occurs in a localized area, such as a city. In such a region, human output may be dominant and may temporarily overload the natural purification scheme of the cycles. The result is an increased concentration of noxious chemicals in the air. The concentrations at which the adverse effects appear will be greater than the concentrations that the pollutants would have in the absence of human activities. The actual concentration need not be large for a substance to be a pollutant; in fact, the numerical value tells us little until we know how much of an increase this represents over the concentration that would occur naturally in the area. According to the passage, human-generated air pollution in localized regions ___.

Xem chi tiết 492 lượt xem 5 năm trước

Clara Barton became known as "The Angel of the Battlefield" during the American Civil War. Born in Oxford, Massachusetts in 1821, Clara Barton's interest in helping soldiers on the battlefield began when she was told army stories from her father. Another event that influenced her decision to help soldiers was an accident her brother had. His injuries were cared for by Barton for 2 years. At the time, she was only Il years old. Barton began teaching school at the age of 15. She taught for 18 years before she moved to Washington, D.C. in 1854. The civil war broke out 6 years later. Immediately, Barton started war service by helping the soldiers with their needs. At the battle of Bull Run, Clara Barton received permission from the government to take care of the sick and hurt. Barton did this with great empathy and kindness. She acknowledged each soldier as a person. Her endurance and courage on the battlefield were admired by many. When the war ended in 1865, she used 4 years of her life to assist the government in searching for soldiers who were missing during the war. The search for missing soldiers and years of hard work made her feeble physically. In 1869, her doctors recommended a trip to Europe for a rest. While she was on vacation, she became involved with the International Red Cross, an organization set up by the Geneva Convention in 1864. Clara Barton realized that the Red Cross would be a big help to the United States. After she returned to the United States, she worked very hard to create an American Red Cross. She talked to government leaders and let American people know about the Red Cross. In 1881, the National Society of the Red Cross was finally established with its headquarters in Washington, D.C. Clara Barton managed its activities for 23 years. Barton never let her age stop her from helping people. At the age of 79, she helped flood victims in Galveston, Texas. Barton finally resigned from the Red Cross in 1904. She was 92 years old and had truly earned her title "The Angel of the Battlefield".   According to paragraph 1, which of the following is true of the young Clara Barton? 

Xem chi tiết 1.3 K lượt xem 5 năm trước

Many scientists believe our love of sugar may actually be an addiction. When we eat or drink sugary foods, the sugar enters our blood and affects parts of our brain that make us feel good. Then the good feeling goes away, leaving us wanting more. All tasty foods do this, but sugar has a particularly strong effect. In this way, it is in fact an addictive drug, one that doctors recommend we all cut down on. "It seems like every time I study an illness and trace a path to the first cause, I find my way back to sugar," says scientist Richard Johnson. One- third of adults worldwide have high blood pressure, and up to 347 million have diabetes. Why? "Sugar, we believe, is one of the culprits, if not the major culprit," says Johnson. Our bodies are designed to survive on very little sugar. Early humans often had very little food, so our bodies learned to be very efficient in storing sugar as fat. In this way, we had energy stored for when there was no food. But today, most people have more than enough. So the very thing that once saved us may now be killing us. So what is the solution? It's obvious that we need to eat less sugar. The trouble is, in today's world, it's extremely difficult to avoid. From breakfast cereals to after-dinner desserts, our foods are increasingly filled with it. Some manufacturers even use sugar to replace taste in foods that are advertised as low in fat. But there are those who are fighting back against sugar. Many schools are replacing sugary desserts with healthier options like fruit. Other schools are growing their own food in gardens, or building facilities like walking tracks so students and others in the community can exercise. The battle has not yet been lost.   Which of the following statements about sugar is NOT true? 

Xem chi tiết 1.5 K lượt xem 5 năm trước

Many scientists believe our love of sugar may actually be an addiction. When we eat or drink sugary foods, the sugar enters our blood and affects parts of our brain that make us feel good. Then the good feeling goes away, leaving us wanting more. All tasty foods do this, but sugar has a particularly strong effect. In this way, it is in fact an addictive drug, one that doctors recommend we all cut down on. "It seems like every time I study an illness and trace a path to the first cause, I find my way back to sugar," says scientist Richard Johnson. One- third of adults worldwide have high blood pressure, and up to 347 million have diabetes. Why? "Sugar, we believe, is one of the culprits, if not the major culprit," says Johnson. Our bodies are designed to survive on very little sugar. Early humans often had very little food, so our bodies learned to be very efficient in storing sugar as fat. In this way, we had energy stored for when there was no food. But today, most people have more than enough. So the very thing that once saved us may now be killing us. So what is the solution? It's obvious that we need to eat less sugar. The trouble is, in today's world, it's extremely difficult to avoid. From breakfast cereals to after-dinner desserts, our foods are increasingly filled with it. Some manufacturers even use sugar to replace taste in foods that are advertised as low in fat. But there are those who are fighting back against sugar. Many schools are replacing sugary desserts with healthier options like fruit. Other schools are growing their own food in gardens, or building facilities like walking tracks so students and others in the community can exercise. The battle has not yet been lost.  According to the passage, why is it so hard to avoid sugar?     

Xem chi tiết 1.1 K lượt xem 5 năm trước

Many scientists believe our love of sugar may actually be an addiction. When we eat or drink sugary foods, the sugar enters our blood and affects parts of our brain that make us feel good. Then the good feeling goes away, leaving us wanting more. All tasty foods do this, but sugar has a particularly strong effect. In this way, it is in fact an addictive drug, one that doctors recommend we all cut down on. "It seems like every time I study an illness and trace a path to the first cause, I find my way back to sugar," says scientist Richard Johnson. One- third of adults worldwide have high blood pressure, and up to 347 million have diabetes. Why? "Sugar, we believe, is one of the culprits, if not the major culprit," says Johnson. Our bodies are designed to survive on very little sugar. Early humans often had very little food, so our bodies learned to be very efficient in storing sugar as fat. In this way, we had energy stored for when there was no food. But today, most people have more than enough. So the very thing that once saved us may now be killing us. So what is the solution? It's obvious that we need to eat less sugar. The trouble is, in today's world, it's extremely difficult to avoid. From breakfast cereals to after-dinner desserts, our foods are increasingly filled with it. Some manufacturers even use sugar to replace taste in foods that are advertised as low in fat.   But there are those who are fighting back against sugar. Many schools are replacing sugary desserts with healthier options like fruit. Other schools are growing their own food in gardens, or building facilities like walking tracks so students and others in the community can exercise. The battle has not yet been lost.  What does the phrase "the very thing" in paragraph 3 refer to? 

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Many scientists believe our love of sugar may actually be an addiction. When we eat or drink sugary foods, the sugar enters our blood and affects parts of our brain that make us feel good. Then the good feeling goes away, leaving us wanting more. All tasty foods do this, but sugar has a particularly strong effect. In this way, it is in fact an addictive drug, one that doctors recommend we all cut down on. "It seems like every time I study an illness and trace a path to the first cause, I find my way back to sugar," says scientist Richard Johnson. One- third of adults worldwide have high blood pressure, and up to 347 million have diabetes. Why? "Sugar, we believe, is one of the culprits, if not the major culprit," says Johnson. Our bodies are designed to survive on very little sugar. Early humans often had very little food, so our bodies learned to be very efficient in storing sugar as fat. In this way, we had energy stored for when there was no food. But today, most people have more than enough. So the very thing that once saved us may now be killing us. So what is the solution? It's obvious that we need to eat less sugar. The trouble is, in today's world, it's extremely difficult to avoid. From breakfast cereals to after-dinner desserts, our foods are increasingly filled with it. Some manufacturers even use sugar to replace taste in foods that are advertised as low in fat. But there are those who are fighting back against sugar. Many schools are replacing sugary desserts with healthier options like fruit. Other schools are growing their own food in gardens, or building facilities like walking tracks so students and others in the community can exercise. The battle has not yet been lost.   The word "culprit" in paragraph 2 is closest in meaning to __________.

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Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 35 to 42. The Nobel Peace Prize is awarded annually and the first woman to win this prize was Baroness Bertha Felicie Sophie von Suttner in 1905. In fact, her work inspired creation of the Prize. The first American woman to win the prize was Jane Addams, in 1931. However, Addams is best known as the founder of Hull House. Jane Addams was born in 1860, into a wealthy family. She was one of a small number of women in her generation to graduate from college. Her commitment to improving the lives of those around her led her to work for social reform and world peace. In the 1880s Jane Addams travelled to Europe. While she was in London, she visited a “settlement house” called Toynbee Hall. Inspired by Toynbee Hall, Addams and her friends, Ellen Gates Starr, opened Hull House in a neighborhood of slums in Chicago in 1899. Hull House provided a day care center for children of working mothers, a community kitchen, and visiting nurses. Addams and her staff gave classes in English literacy, art and other subjects. Hull House also became a meeting place for clubs and labor unions. Most of the people who worked with Addams in Hull House were well educated, middle-class women. Hull House gave them an opportunity to use their education and it provided a training ground for careers in social work. Before World War I, Addams was probably the most beloved woman in America. In a newspaper poll that asked, “Who among our contemporaries are of the most value to the community?” Jane Addams was rated second, after Thomas Edison. When she opposed America’s involvement in World War I, however, newspaper editors called her a traitor and a fool, but she never changed her mind. Jane Addams was a strong champion of several other causes. Until 1920, American women could not vote. Addams joined in the movement for women’s suffrage and was a 7 vice president of the National American Woman Suffrage Association. She was a founding member of the National Association for the Advancement of Colored People (NAACP), and was president of the Women’s International League for Peace and Freedom. Her reputation was gradually restored during the last years of her life. She died of cancer in 1935. In paragraph three, the word “contemporaries” is closest in meaning to _________ .

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Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 35 to 42. The Nobel Peace Prize is awarded annually and the first woman to win this prize was Baroness Bertha Felicie Sophie von Suttner in 1905. In fact, her work inspired creation of the Prize. The first American woman to win the prize was Jane Addams, in 1931. However, Addams is best known as the founder of Hull House. Jane Addams was born in 1860, into a wealthy family. She was one of a small number of women in her generation to graduate from college. Her commitment to improving the lives of those around her led her to work for social reform and world peace. In the 1880s Jane Addams travelled to Europe. While she was in London, she visited a “settlement house” called Toynbee Hall. Inspired by Toynbee Hall, Addams and her friends, Ellen Gates Starr, opened Hull House in a neighborhood of slums in Chicago in 1899. Hull House provided a day care center for children of working mothers, a community kitchen, and visiting nurses. Addams and her staff gave classes in English literacy, art and other subjects. Hull House also became a meeting place for clubs and labor unions. Most of the people who worked with Addams in Hull House were well educated, middle-class women. Hull House gave them an opportunity to use their education and it provided a training ground for careers in social work. Before World War I, Addams was probably the most beloved woman in America. In a newspaper poll that asked, “Who among our contemporaries are of the most value to the community?” Jane Addams was rated second, after Thomas Edison. When she opposed America’s involvement in World War I, however, newspaper editors called her a traitor and a fool, but she never changed her mind. Jane Addams was a strong champion of several other causes. Until 1920, American women could not vote. Addams joined in the movement for women’s suffrage and was a 7 vice president of the National American Woman Suffrage Association. She was a founding member of the National Association for the Advancement of Colored People (NAACP), and was president of the Women’s International League for Peace and Freedom. Her reputation was gradually restored during the last years of her life. She died of cancer in 1935. According to the passage, all of the following statements about Jane Addams are true EXCEPT ___________.

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  Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 35 to 42. The Nobel Peace Prize is awarded annually and the first woman to win this prize was Baroness Bertha Felicie Sophie von Suttner in 1905. In fact, her work inspired creation of the Prize. The first American woman to win the prize was Jane Addams, in 1931. However, Addams is best known as the founder of Hull House. Jane Addams was born in 1860, into a wealthy family. She was one of a small number of women in her generation to graduate from college. Her commitment to improving the lives of those around her led her to work for social reform and world peace. In the 1880s Jane Addams travelled to Europe. While she was in London, she visited a “settlement house” called Toynbee Hall. Inspired by Toynbee Hall, Addams and her friends, Ellen Gates Starr, opened Hull House in a neighborhood of slums in Chicago in 1899. Hull House provided a day care center for children of working mothers, a community kitchen, and visiting nurses. Addams and her staff gave classes in English literacy, art and other subjects. Hull House also became a meeting place for clubs and labor unions. Most of the people who worked with Addams in Hull House were well educated, middle-class women. Hull House gave them an opportunity to use their education and it provided a training ground for careers in social work. Before World War I, Addams was probably the most beloved woman in America. In a newspaper poll that asked, “Who among our contemporaries are of the most value to the community?” Jane Addams was rated second, after Thomas Edison. When she opposed America’s involvement in World War I, however, newspaper editors called her a traitor and a fool, but she never changed her mind. Jane Addams was a strong champion of several other causes. Until 1920, American women could not vote. Addams joined in the movement for women’s suffrage and was a 7 vice president of the National American Woman Suffrage Association. She was a founding member of the National Association for the Advancement of Colored People (NAACP), and was president of the Women’s International League for Peace and Freedom. Her reputation was gradually restored during the last years of her life. She died of cancer in 1935. According to the passage, Jane Addams’ reputation was damaged when she _____.  

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Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 35 to 42. The Nobel Peace Prize is awarded annually and the first woman to win this prize was Baroness Bertha Felicie Sophie von Suttner in 1905. In fact, her work inspired creation of the Prize. The first American woman to win the prize was Jane Addams, in 1931. However, Addams is best known as the founder of Hull House. Jane Addams was born in 1860, into a wealthy family. She was one of a small number of women in her generation to graduate from college. Her commitment to improving the lives of those around her led her to work for social reform and world peace. In the 1880s Jane Addams travelled to Europe. While she was in London, she visited a “settlement house” called Toynbee Hall. Inspired by Toynbee Hall, Addams and her friends, Ellen Gates Starr, opened Hull House in a neighborhood of slums in Chicago in 1899. Hull House provided a day care center for children of working mothers, a community kitchen, and visiting nurses. Addams and her staff gave classes in English literacy, art and other subjects. Hull House also became a meeting place for clubs and labor unions. Most of the people who worked with Addams in Hull House were well educated, middle-class women. Hull House gave them an opportunity to use their education and it provided a training ground for careers in social work. Before World War I, Addams was probably the most beloved woman in America. In a newspaper poll that asked, “Who among our contemporaries are of the most value to the community?” Jane Addams was rated second, after Thomas Edison. When she opposed America’s involvement in World War I, however, newspaper editors called her a traitor and a fool, but she never changed her mind. Jane Addams was a strong champion of several other causes. Until 1920, American women could not vote. Addams joined in the movement for women’s suffrage and was a 7 vice president of the National American Woman Suffrage Association. She was a founding member of the National Association for the Advancement of Colored People (NAACP), and was president of the Women’s International League for Peace and Freedom. Her reputation was gradually restored during the last years of her life. She died of cancer in 1935. Jane Addams was inspired to open Hull House because ________.

Xem chi tiết 1.5 K lượt xem 5 năm trước

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 35 to 42. The Nobel Peace Prize is awarded annually and the first woman to win this prize was Baroness Bertha Felicie Sophie von Suttner in 1905. In fact, her work inspired creation of the Prize. The first American woman to win the prize was Jane Addams, in 1931. However, Addams is best known as the founder of Hull House. Jane Addams was born in 1860, into a wealthy family. She was one of a small number of women in her generation to graduate from college. Her commitment to improving the lives of those around her led her to work for social reform and world peace. In the 1880s Jane Addams travelled to Europe. While she was in London, she visited a “settlement house” called Toynbee Hall. Inspired by Toynbee Hall, Addams and her friends, Ellen Gates Starr, opened Hull House in a neighborhood of slums in Chicago in 1899. Hull House provided a day care center for children of working mothers, a community kitchen, and visiting nurses. Addams and her staff gave classes in English literacy, art and other subjects. Hull House also became a meeting place for clubs and labor unions. Most of the people who worked with Addams in Hull House were well educated, middle-class women. Hull House gave them an opportunity to use their education and it provided a training ground for careers in social work. Before World War I, Addams was probably the most beloved woman in America. In a newspaper poll that asked, “Who among our contemporaries are of the most value to the community?” Jane Addams was rated second, after Thomas Edison. When she opposed America’s involvement in World War I, however, newspaper editors called her a traitor and a fool, but she never changed her mind. Jane Addams was a strong champion of several other causes. Until 1920, American women could not vote. Addams joined in the movement for women’s suffrage and was a 7 vice president of the National American Woman Suffrage Association. She was a founding member of the National Association for the Advancement of Colored People (NAACP), and was president of the Women’s International League for Peace and Freedom. Her reputation was gradually restored during the last years of her life. She died of cancer in 1935. The word “commitment” in the second paragraph is closest in meaning to _______.

Xem chi tiết 1.1 K lượt xem 5 năm trước

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 30 to 34. An air pollutant is defined as a compound added directly or indirectly by humans to the atmosphere in such quantities as to affect humans, animals, vegetation, or materials adversely. Air pollution requires a very flexible definition that permits continuous change. When the first air pollution laws were established in England in the fourteenth century, air pollutants were limited to compounds that could be seen or smelled - a far cry from the extensive list of harmful substances known today. As technology has developed and knowledge of the health aspects of various chemicals has increased, the list of air pollutants has lengthened. In the future, even water vapor might be considered an air pollutant under certain conditions. Many of the more important air pollutants, such as sulfur oxides, carbon monoxide, and nitrogen oxides, are found in nature. As the Earth developed, the concentration of these pollutants was altered by various chemical reactions; they became components in biogeochemical cycles. These serve as an air purification scheme by allowing the compounds to move from the air to the water or soil. On a global basis, nature's output of these compounds dwarfs that resulting from human activities. However, human production usually occurs in a localized area, such as a city. In such a region, human output may be dominant and may temporarily overload the natural purification scheme of the cycles. The result is an increased concentration of noxious chemicals in the air. The concentrations at which the adverse effects appear will be greater than the concentrations that the pollutants would have in the absence of human activities. The actual concentration need not be large for a substance to be a pollutant; in fact, the numerical value tells us little until we know how much of an increase this represents over the concentration that would occur naturally in the area. The word “These” in the second paragraph is closest in meaning to _______.

Xem chi tiết 2.2 K lượt xem 5 năm trước

  Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 30 to 34. An air pollutant is defined as a compound added directly or indirectly by humans to the atmosphere in such quantities as to affect humans, animals, vegetation, or materials adversely. Air pollution requires a very flexible definition that permits continuous change. When the first air pollution laws were established in England in the fourteenth century, air pollutants were limited to compounds that could be seen or smelled - a far cry from the extensive list of harmful substances known today. As technology has developed and knowledge of the health aspects of various chemicals has increased, the list of air pollutants has lengthened. In the future, even water vapor might be considered an air pollutant under certain conditions. Many of the more important air pollutants, such as sulfur oxides, carbon monoxide, and nitrogen oxides, are found in nature. As the Earth developed, the concentration of these pollutants was altered by various chemical reactions; they became components in biogeochemical cycles. These serve as an air purification scheme by allowing the compounds to move from the air to the water or soil. On a global basis, nature's output of these compounds dwarfs that resulting from human activities. However, human production usually occurs in a localized area, such as a city. In such a region, human output may be dominant and may temporarily overload the natural purification scheme of the cycles. The result is an increased concentration of noxious chemicals in the air. The concentrations at which the adverse effects appear will be greater than the concentrations that the pollutants would have in the absence of human activities. The actual concentration need not be large for a substance to be a pollutant; in fact, the numerical value tells us little until we know how much of an increase this represents over the concentration that would occur naturally in the area. It can be inferred from the first paragraph that __________.  

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Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 30 to 34. An air pollutant is defined as a compound added directly or indirectly by humans to the atmosphere in such quantities as to affect humans, animals, vegetation, or materials adversely. Air pollution requires a very flexible definition that permits continuous change. When the first air pollution laws were established in England in the fourteenth century, air pollutants were limited to compounds that could be seen or smelled - a far cry from the extensive list of harmful substances known today. As technology has developed and knowledge of the health aspects of various chemicals has increased, the list of air pollutants has lengthened. In the future, even water vapor might be considered an air pollutant under certain conditions. Many of the more important air pollutants, such as sulfur oxides, carbon monoxide, and nitrogen oxides, are found in nature. As the Earth developed, the concentration of these pollutants was altered by various chemical reactions; they became components in biogeochemical cycles. These serve as an air purification scheme by allowing the compounds to move from the air to the water or soil. On a global basis, nature's output of these compounds dwarfs that resulting from human activities. However, human production usually occurs in a localized area, such as a city. In such a region, human output may be dominant and may temporarily overload the natural purification scheme of the cycles. The result is an increased concentration of noxious chemicals in the air. The concentrations at which the adverse effects appear will be greater than the concentrations that the pollutants would have in the absence of human activities. The actual concentration need not be large for a substance to be a pollutant; in fact, the numerical value tells us little until we know how much of an increase this represents over the concentration that would occur naturally in the area. The word "adversely" in the first paragraph is closest in meaning to _______.

Xem chi tiết 2.2 K lượt xem 5 năm trước