题目内容：

 

What makes it rain? Rain falls from clouds for the same reason anything falls to Earth. The Earth's gravity pulls it. But every cloud is made of water droplets or ice crystal(冰晶). Why doesn't rain or snow fall constantly from all clouds? The droplets or ice crystals in clouds are extremely small. The effect of gravity on them is slight. Air currents move and lift droplets so that the net downward amount of water is zero, even though the droplets are moving constantly.

    It can be seen that droplets and ice crystals behave somewhat like dust in the air in a beam of sunlight. To the casual observer, dust seems to act in a totally random fashion, moving about

without fixed direction. But in fact dust particles（分子） are much larger than water droplets and they finally fall. The average size of a cloud droplet is only 0.0004 inch in diameter（直径）. It is so small that it would take sixteen hours to fall half a mile in perfectly still air, and it doesn't fall out of moving air at all. Only when the droplet grows to a diameter of 0.008 inch or larger can it fall from the cloud. The average raindrop contains a million times as much water as a tiny cloud droplet. The growth of a cloud droplet to a size larger enough to fall out is the cause of rain and other forms of precipitation. This important growth process is called "coalescence".

1. Ice crystals do NOT immediately fall to Earth because            .

A. they are kept up by air currents

B. most of them change into steam

C. they combine with other chemicals in the atmosphere

D. their electrical charges draw them away from the earth

2. The underlined word "random" in line 10 most probably means            .

A. unpredictable     B. strict    C. independent     D. abnormal

3. What can be inferred about drops of water larger than 0.008 inch in diameter?

A. They never occur.

B. They are not affected by the force of gravity.

C. In moving air they would fall to earth.

D.In still air they fall a speed of thirty-two miles per hour.

4. How much bigger is the rain drop than a loud droplet?

A. 200 times bigger

B. 1,000 times bigger

C. 100,000 times bigger

     D. 1,000,000 times bigger