阅读理解。
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
hot sun make the earth even drier. Plants don't let the wind blow the dirt away. When a little bit of rain falls, the
plants hold the water. Without plants, the land can become a desert much more easily.
1. Deserts _________. [ ]
A. get very little rain
B. never have any plants or animals in them
C. can all be turned into good land in the future
D. both A and C
2. Small green plants are very important to day 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 the water
D. A, B and C
3. Land is becoming desert little by little because ________. [ ]
A. plants can't grow there
B. there's not enough rain
C. people are doing bad things to the earth.
D. scientists know little about desert
4. Which is the main idea of the first paragraph? [ ]
A. Scientists know how to change land 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 the desert, people can live there.
D. More and more places are becoming deserts all the time.
5. After reading this, we learn that ________. [ ]
A. plant can keep dry land from becoming desert.
B. it is good to get rid of (清除) the grass in the desert
C. all places without much rain will become deserts
D. it is better to grow crops on dry land than to grow grass
处由静止开始释放，导体棒在压缩弹簧的作用下向左运动，当导体棒运动到O点时弹簧与导体棒分离，导体棒由MN运动到O点的过程中做匀速直线运动，导体棒始终与MN平行。已知导体棒与弹簧彼此绝缘，导体棒和导轨单位长度的电阻均为r0，弹簧被压缩后所获得的弹性势能可用公式Ep=
kx2计算，k为弹簧的劲度系数，x为弹簧的形变量。
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