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三、阅读理解（共7小题；每小题2分，满分14分）

　　The earth is the only planet that scientists think has life. Why does the earth have life while the other planets don't？For one thing，the earth is just the right temperature. As the third planet from the sun，the earth seems to be just the right distance away. The planets which are closer to the sun are so hot that their surfaces bake(烘烤)in the sun. The farthest planets are cold balls.

　　When the earth developed--which scientists believe may have happened about 4 billion years ago--many gases covered the earth. The gases caused the earth to be hot. But something wonderful happened. The temperature was just right for thick clouds to form. It rained very hard for a very long time. This gave the earth its oceans. Water made it possible for plants to grow. The plants created oxygen in the atmosphere. Oxygen is the gas that humans and animals breathe.

　　Only one other planet in the solar system seems to be something like the earth. That planet is Mars (火星). Mars is smaller than the earth，and it is quite a bit cooler. But it is not too cold for humans. On some days，the temperatures are as low as a winter day in the northern United States. If you wore a special spacesuit，you could walk around on Mars. You would have to bring your own air to breathe，because the air on Mars is too thin to breathe.

　　Mars has the largest volcano(火山) in the solar system. It is sixteen miles high. The highest volcano on the earth is five miles high. The most unexpected(未预料到的) sight on Mars is dried­up river beds. Scientists believe that Mars was once much better than it is now. Does this mean there could have been living things on Mars？Scientists are not sure，but there has been no sign so far.

　　21. Which of the following is NOT the reason that the earth has life?

　　A. The earth is just the right temperature.

　　B. The volcanoes on the earth are lower than those on Mars.

　　C. There is oxygen in the atmosphere of the earth.

　　D. The earth seems to be just the right distance away from the sun.

　　22. The underlined phrase "the third planet" in the first paragraph refers to_______.

　　A. the Sun B. the Moon

　　C. the Earth D. the Mars

　　23. Which of the following about the Mars is NOT true?

　　A. There could have been living things on Mars.

　　B. Mars has the largest volcano in the solar system.

　　C. There is almost no oxygen in the air on Mars.

　　D. Mars is smaller than the earth，and it is quite a bit cooler.

Damage to Ozone layer Gets Worse

　　In the middle of winter, when snow is falling in many parts of the United States, scientists have sounded a warning to people who plan to spend many hours in the sun this summer. The warning is: The sun's summertime rays are more dangerous than once thought.

A team of scientists from 80 nations recently reported to the United Nations that a layer of ozone (臭氧层) in the atmosphere, which protects humans from harmful levels of ultraviolet（紫外线） radiation, will be thinner over the United States this summer. The thinner layer allows more ultraviolet rays from the sun to reach Earth. The extra amount of ultraviolet radiation could cause an increase in the number of cases of skin cancer.