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

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 .

1.It is the       on the earth that makes life possible .

A．temperature       B．water             C．oxygen           D．temperature , water and oxygen

2.The underlined phrase “ the third planet ” means       .

A．the sun           B．the earth          C．the moon         D．Mars

3.Which planet in the solar system is most like the earth ?

A．Mars .            B．Moon .            C．Sun .             D．Mars and Moon .

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 .

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