The earth is the only planet that scientists are certain has life .What does the earth have that 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 that are closer to the sun are so hot that their surfaces bake in the sun . The farthest planets are cold balls .
When the earth developed— which scientists believe may have happened about billions of 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 . 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 cold 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 , though . The air on Mars is too thin to breathe . Mars has the largest volcano 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 dried-up river beds . Scientists believe that Mars was once much wetter that 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:It is the on the earth that makes life possible .
A．temperature
B．water
C．oxygen
D．temperature , water and oxygen小题2:The underlined phrase “ the third planet ” means .
A．the sun
B．the earth
C．the moon
D．Mars小题3:Which planet in the solar system is most like the earth ?
A．Mars .
B．Moon .
C．Sun .
D．Mars and Moon .小题4:Which of the following is TRUE ?
A．The earth has a history of about 4 million years .
B．There is no air on Mars .
C．The largest volcano on Mars is over three times higher than the highest one on the earth
D．Scientists believe there are living things on Mars .小题5:The best title for the passage should be .
A．Living Things on Our Earth
B．The Moon
C．Living Things on the Moon
D．Mars Is Most Like the Earth
