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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 is not mentioned in the passage?
选项 A、Rain does not fall from all the clouds.
B、The effect of the gravity on droplets is minute.
C、The average raindrop contains much more water than a tiny cloud droplets.
D、The shape of the clouds have a great influence on the raindrop.
答案D
解析 从第一段的第五句话“为什么雨或雪不是不断地从所有的云中落下呢?”可知选项A正确，从第一段第七句“重力对它们的影响是很小的”可知选项B正确：从第二段第七句“一般的雨滴所包含的水比云中的一个微小的液滴所包含的水要多一百万倍”可知选项C正确。而选项D在本文中未提及，故选D。
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