The earth is the only planet that scientists think has life. Why does the earth have life while the other planets (行星) 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 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 space suit, you could walk around on Mars. You would have to bring your own air to breathe, 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 (未预测到的) sight on Mars is dried up 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. 小题1:Which of the following is NOT the reason that the earth has life?
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