单选题 Scientists are trying to make the deserts into good land again. They want to bring water to the deserts, so people can live and grow food. They are learning a lot about the deserts. But more and more of the earth is becoming desert all the time. Scientists may not be able to change the desert. Scientists think that people make deserts. People are doing bad things to the earth.
Some places on the earth don't get very much rain. But they still don't become deserts. This is because some green plants are growing there. Small green plants and grass are very important to dry places. Plants don't let the hot sun make the earth even drier. Plants don't let the wind blow the dirt(土)away. When a little bit of rain falls, the plants hold the water. Without plants, the land can become a desert much more easily.

单选题 Deserts ______.
A. get very little rain
B. never have any plants or animals in them
C. can all be turned into good land before long
D. both A and C

单选题 Small green plants are very important to dry places because ______.
A. they don't let the sun make the earth even drier
B. they don't let the wind blow the earth away
C. they hold water
D. all of the above

单选题 Land is becoming desert little by little because ______.
A. plants can't grow there
B. there is not enough rain
C. people haven't done what scientists wish them to do
D. scientists know little about the deserts

单选题 Which is the main idea of the first paragraph?
A. Scientists know how to change desert into good land.
B. Land is becoming desert faster than scientists can change it back into good land.
C. If scientists can bring water to desert, people can live and grow food theory.
D. More and more places are becoming deserts all the time.