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．
16．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
17．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．
18．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
19．After reading this passage， we learn that ＿＿＿＿＿＿＿＿ ．
A．plants can keep dry land from becoming desert
B．it is good to get rid of the grass in the deserts
C．all places without much rain will become deserts
D．it is better to grow crops on dry land than to cut them
20．Why is more and more land becoming desert according to this passage？
A．Earthquakes make deserts. B. Wind makes deserts.
C. Lack of water makes deserts. D. People make deserts．