问题 多项选择题

弹簧式安全阀的工作原理通过弹簧的弹力抵压阀瓣使之与阀座密封面密合,具体的就是(),从而防止设备或管路的超压,保证安全生产。

A.当介质压力高于弹簧的弹性力时,弹簧被压缩,介质自行排出

B.当介质压力低于弹簧的弹性力时,弹簧被压缩,介质自行排出

C.当介质的压力升到与弹簧的弹性力相等时,密封面又开始重新密合

D.当介质的压力降到与弹簧的弹性力相等时,密封面又开始重新密合

答案

参考答案:A, D

阅读理解

阅读理解。

     Forests have always been useful and important to men who make use of them in many ways. Every day

trees are serving man everywhere. Trees supply man with fruits and building materials in the form of wood.

Without trees it would be impossible to build houses, boats, bridges and so on. Furniture such as desks, chairs

and beds is made of wood. Trees can stop man from the terrible heat. They're also useful in preventing (阻止)

good and rich top soil from being washed away during heavy rains. If there were no forests, heavy rains would wash away the rich surface (表层) soil that is so important to plants. The result is that the land will become a

desert. There are plenty of desert areas in the world. A long time ago these desert areas used to be very rich

areas, but men in the past had no enough knowledge about science of nature, and they cut down too many

trees in the area where they lived and never planted new ones. By and by, the rich surface soil was blown and

washed away by strong winds and heavy rains. In the end the rich land changed into useless deserts where

nothing could grow.

 根据短文内容,判断正(T)误(F).

(     )1. Trees are not serving man everywhere.

(     )2. Without trees it would be possible to build houses, boats, bridges and so on.

( )3. Trees can stop man from the terrible heat.

( )4. There are few desert areas in the world.

(     )5. In the end the rich land changed into useless deserts where nothing could grow.

单项选择题