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  3. What makes it rain? Rain falls from clouds for the same reason anything falls to Earth. The Earth’s Gravity(重力) pulls it. But ev

What makes it rain? Rain falls from clouds for the same reason anything falls to Earth. The Earth’s Gravity(重力) pulls it. But ev

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问题     What makes it rain? Rain falls from clouds for the same reason anything falls to Earth. The Earth’s Gravity(重力) pulls it. But every cloud is made of water droplets or ice crystals. Why doesn’t rain or snow fall constantly from all clouds? The droplets or ice crystals in clouds are exceedingly small. The effect of gravity on them is minute. Air currents move and lift droplets so that the net downward displacement is zero, even though the droplets are in constant motion.
    Droplets and ice crystals behave somewhat like dust in the air made visible in a shaft(束) of sunlight. To the casual observer, dust seems to act in a totally random fashion, moving about wildly without fixed direction. But in fact dust particles are much larger than water droplets and they finally fall. The cloud droplet of average size is only 1/2,500 inch in diameter. (78)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 form 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 large enough to fall out is the cause of rain.
What can be inferred about drops of water larger than 1/125 inch in diameter?
选项 A、They never occur.
B、They are not affected by the force of gravity.
C、They would fall to earth.
D、In moving air they fall at a speed of thirty-two miles per hour.
答案C
解析 从第二段第六句话可知，只有当雨滴直径达到1／125英寸或更大时，它才会从云层中降落。
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