题目内容：

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 in time．

Why is more and more land becoming desert？ Scientists think that people make deserts． People are doing bad things to the earth．

Some places on the earth don't get 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 sun make the earth even drier． Plants do not let the wind blow the dirt away． When a bit of rain falls， the plants hold the water． Without plants， the land can become desert more easily．

1.Deserts _______．

A． never have any plants or animals in them

B． can all be turned into good land before long

C． are becoming smaller and smaller

D． get very little rain

2.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 soil away

C． they hold water

D． All of the above．

3.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

4.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    there．

D． More and more places are becoming deserts all the time．