15000 bài tập tách từ đề thi thử môn Tiếng Anh có đáp án (Phần 3)

Quite different from storm surges are the giant sea waves called tsunamis, which derive their name from the Japanese expression for “high water in a harbor.” These waves are also referred to by the general public as tidal waves, although they have relatively little to do with tides. Scientists often referred to them as seismic sea waves, far more appropriate in that they do result from undersea seismic activity.

    Tsunamis are caused when the sea bottom suddenly moves, during an underwater earthquake or volcano for example, and the water above the moving earth is suddenly displaced. This sudden shift of water sets off a series of waves. These waves can travel great distances at speeds close to 700 kilometers per hour. In the open ocean, tsunamis have little noticeable amplitude, often no more than one or two meters. It is when they hit the shallow waters near the coast that they increase in height, possibly up to 40 meters.

    Tsunamis often occur in the Pacific because the Pacific is an area of heavy seismic activity. Two areas of the Pacific well accustomed to the threat of tsunamis are Japan and Hawaii. Because the seismic activity that causes tsunamis in Japan often occurs on the ocean bottom quite close to the islands, the tsunamis that hit Japan often come with little warning and can, therefore, prove disastrous. Most of the tsunamis that hit the Hawaiian Islands, however, originate thousands of miles away near the coast of Alaska, so these tsunamis have a much greater distance to travel and the inhabitants of Hawaii generally have time for warning of their imminent arrival.

    Tsunamis are certainly not limited to Japan and Hawaii. In 1755, Europe experienced a calamitous tsunami, when movement along the fault lines near the Azores caused a massive tsunami to sweep onto the Portuguese coast and flood the heavily populated area around Lisbon. The greatest tsunami on record occurred on the other side of the world in 1883 when the Krakatoa volcano underwent a massive explosion, sending waves more than 30 meters high onto nearby Indonesian islands; the tsunami from this volcano actually traveled around the world and was witnessed as far away as the English Channel.

Millions of people are using cellphones today. In many places, it is actually considered unusual not to use one. In many countries, cellphones are very popular with young people. They find that the phones are more than a means of communication - having a mobile phone shows that they are cool and connected.

    The explosion in mobile phone use around the world has made some health professionals worried. Some doctors are concerned that in the future many people may suffer health problems from the use of mobile phones. In England, there has been a serious debate about this issue. Mobile phone companies are worried about the negative publicity of such ideas. They say that there is no proof that mobile phones are bad for your health.

    On the other hand, medical studies have shown changes in the brain cells of some people who use mobile phones. Signs of change in the tissues of the brain and head can be detected with modern scanning equipment. In one case, a traveling salesman had to retire at young age because of serious memory loss. He couldn't remember even simple tasks. He would often forget the name of his own son. This man used to talk on his mobile phone for about six hours a day, every day of his working week, for a couple of years. His family doctor blamed his mobile phone use, but his employer's doctor didn't agree.

    What is it that makes mobile phones potentially harmful? The answer is radiation. High-tech machines can detect very small amounts of radiation from mobile phones. Mobile phone companies agree that there is some radiation, but they say the amount is too small to worry about.

    As the discussion about their safety continues, it appears that it's best to use mobile phones less often. Use your regular phone if you want to talk for a long time. Use your mobile phone only when you really need it. Mobile phones can be very useful and convenient, especially in emergencies. In the future, mobile phones may have a warning label that says they are bad for your health. So for now, it's wise not to use your mobile phone too often.

Traditionally in America, helping the poor was a matter for private charities or local governments. Arriving immigrants depended mainly on predecessors from their homeland to help them start a new life. In the late 19th and early 20th centuries, several European nations instituted public-welfare programs. But such a movement was slow to take hold in the United States because the rapid pace of industrialization and the ready availability of farmland seemed to confirm the belief that anyone who was willing to work could find a job.

    Most of the programs started during the Depression era were temporary relief measures, but one of the programs - Social Security - has become an American institution. Paid for by deductions from the paychecks of working people, Social Security ensures that retired persons receive a modest monthly income and also provides unemployment insurance, disability insurance, and other assistance to those who need it. Social Security payments to retired persons can start at age 62, but many wait until age 65, when the payments are slightly higher. Recently, there has been concern that the Social Security fund may not have enough money to fulfill its obligations in the 21st century, when the population of elderly Americans is expected to increase dramatically. Policy makers have proposed various ways to make up the anticipated deficit, but a long-term solution is still being debated.

    In the years since Roosevelt, other American presidents have established assistance programs. These include Medicaid and Medicare; food stamps, certificates that people can use to purchase food; and public housing which is built at federal expense and made available to persons on low incomes.

    Needy Americans can also turn to sources other than the government for help. A broad spectrum of private charities and voluntary organizations is available. Volunteerism is on the rise in the United States, especially among retired persons. It is estimated that almost 50 percent of Americans over age 18 do volunteer work, and nearly 75 percent of U.S. households contribute money to charity.