题目内容
阅读下列短文, 从所给的四个选项中, 选出最佳答案。
What makes it rain? Rain falls from clouds for the same reason that anything falls to earth. The earth's gravity(引力)pulls it. But every cloud is made of water droplets(微滴)or ice crystals(结晶). Why doesn't rain or snow fall continuously from all clouds? The droplets or ice crystals in clouds are very small. The effect of gravity on them is tiny.
Droplets and ice crystals act like dust in the air which is made visible in the rays of sunlight. To the careless observer, dust seems to act in a totally disorderly way, moving about freely without fixed direction. But in fact dust particles(微粒)are much larger than water droplets and they finally fall. The cloud droplet of ordinary size is only 1/2500 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 does not fall out of moving air at all. Only when the droplet grows to a diameter of 1/125 inch or larger can it fall from the cloud. The ordinary raindrop has a million times as much water as a tiny cloud droplet. The growth of a cloud droplet to a size large enough to fall out is the cause of rain.
(1) Why don't all ice crystals in clouds immediately fall to earth?
[ ]
A. They are hardly affected by the force of gravity.
B. The earth's gravity keeps them from falling.
C. The clouds prevent them from falling.
D. The heat from the sun's rays makes them turn into steam.
(2) What conclusion can we draw from drops of water larger than 1/120 inch in diameter?
[ ]
A. They fall to the ground slowly.
B. In moving air they fall at a speed of thirty-two miles per hour
C. In still air they would fall to earth.
D. The earth's gravity has no effect on them.
(3) If air kept perfectly still for forty-eight hours, a cloud droplet would fall ________.
[ ]
A. 1/2 miles
B. 3/2 miles
C. 2 miles
D. 5/2 miles
(4) What is the main topic of this passage?
[ ]
A. Types of droplets
B. Droplets and ice crystals
C. How gravity affects clouds
D. The cause of rain
(5) According to the passage, which of the following is not true?
[ ]
A. Cloud droplets can fall to earth as dust particles.
B. Though dust particles are very small, we can see them moving in the rays of sunlight.
C. Cloud droplets don't fall at all in moving air.
D. If a cloud droplet grows to a size large enough, it will fall to earth.