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　　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(未預料到的)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.

　　文章大意：文章介紹了地球存在生命的原因以及火星的情況。

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

　　A.The earth is just the right temperature.

　　B.The volcanos 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.

　　答案：B　細節(jié)理解題。文中提到地球是可證實的唯一有生命存在的太陽系行星，其原因包括地球溫度合適、距太陽距離適中和大氣里含有氧氣等。B項火山與此無關。

　　2.The underlined phrase“the third planet”in the first paragraph refers to________.

　　A.the Sun B.the Moon

　　C.the Earth D.the Mars

　　答案：C　詞義猜測題。根據(jù)第一段中內(nèi)容可知距離太陽第三遠的行星指的是地球。

　　3.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.

　　答案：A　細節(jié)理解題。文章最后一段提到火星上有干涸的河床的跡象，但這一點并不能證實火星上存在生命，因為還沒有生命跡象，故A項不正確。其他三項都與文章相符。

　　4.Which of the following is probably the best title for the passage?

　　A.The Earth

　　B.The Moon

　　C.Planets in the Solar System

　　D.Mars is Most Like the Earth

　　答案：D　主旨大意題。文章先說明地球上存在生命的原因又對比了火星和地球，說明火星是與地球最相似的行星。D項做標題最為恰當。