A
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 has 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 because their surfaces are baked(炙烤) 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.
Another planet in the solar system seems to be something like the earth.That planet is Mars.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，as 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 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.
【语篇导读】 本文是一篇说明文。文章先说明地球上存在生命的原因；又对比了火星与地球，说明火星与地球最相似。
1．What is it on the earth that makes life possible?
A．Temperature.
B．Water.
C．Oxygen.
D．Temperature，water and oxygen.
D [细节理解题。根据第二段中“The temperature was just right for...Water made it possible for plants go grow...the gas that humans and animals breathe.”可知，温度、 水和氧气使地球上出现生命成为可能，故选D项。]
2．What does the underlined phrase “the third planet” in the first paragraph refer to?
A．The sun. B．The moon.
C．The earth. D．The Mars.
C [细节理解题。根据常识及画线词所在句后一分句均可知，距离太阳第三远的行星指的是地球，故选C项。]
3．What should be the best title for the passage?
A．Living Things on Our Earth
B．The Moon
C．Living Things on the Moon
D．Mars is Most Like the Earth
D [标题归纳题。本文先讲述的是地球适合人类生存的原因，后面对比火星和地球在环境方面的异同之处，从气温、水、空气等方面展开，具体说明了火星与地球最相似，故选D项。]
