Eventually, a different kind of animal evolves. Sometimes many of the animals on Earth go extinct at the (42) ______ time. Scientists call this a mass extinction.

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Books which give instructions on how to do things are very popular in the United States today. Thousands of these How-to books are useful. In fact, there are about four thousand books with titles that begin with the words “How to”. One book may tell you how to earn more money. Another may tell you how to save or spend it and another may explain how to give your money away.

Many How-to books give advice on  careers. They tell you how to choose a career and how to succeed in it. If you fail, however, you can buy the book “How to Turn Failure into Success”. If you would like to become very rich, you can buy the book “How to Make a Millionaire”. If you never make any money at all, you may need a book called “How to Live on Nothing”.

One of the most popular types of books is one that helps you with personal problems. If you want to have a better love of life, you can read “How to Succeed in Love every Minute of Your Life”. If you are tired of books on happiness, you may prefer books which give step-by-step instructions on how to redecorate or enlarge a house.

Why have How-to books become so popular? Probably because life has become so complex. Today people have far more free time to use, more choices to make, and more problems to solve. How-to books help people deal with modern life.

What is the passage mainly about?

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 39 to 43

WHY DO ANIMALS GO EXTINCT?

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Most desert animals will drink water if confronted with it, but many of them never have any opportunity. All living things must have water, or they will expire. The herbivores find it in desert plants. The carnivores slake their thirst with the flesh and blood of living prey. One of the most remarkable adjustments, however, has been made by the tiny kangaroo rat, who not only lives without drinking but subsists on a diet of dry seeds containing about 5% free water. Like other animals, he has the ability to manufacture water in his body by a metabolic conversion of carbohydrates. But he is notable for the parsimony with which he conserves his small supply by every possible means, expending only minuscule amounts in his excreta and through evaporation from his respiratory tract.

Investigation into how the kangaroo rat can live without drinking water has involved various experiments with these small animals. Could kangaroo rats somehow store water in their bodies and slowly utilize these resources in the long periods when no free water is available from dew or rain? The simplest way to settle this question was to determine the total water content in the animals to see if it decreases as they are kept for long periods on a dry diet. If they slowly use up their water, the body should become increasingly dehydrated, and if they begin with a store of water, this should be evident from an initial high water content. Results of such experiments with kangaroo rats on dry diets for more than 7 weeks showed that the rats maintained their body content during the long period of water deprivation. When the kangaroo rats were given free access to water, they did not drink water. They did nibble on small pieces of watermelon, but this did not change appreciably the water intent in their bodies, which remained at 66.3% to 67.2% during this period.

This is very close to the water content of dry-fed animals (66.5%), and the availability of free water, therefore, did not lead to any “storage” that could be meaningful as a water reserve. This makes it reasonable to conclude that physiological storage of water is not a factor in the kangaroo rat’s ability to live on dry food.

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