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".

　　68. 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

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

　　.

　　A. unpredictable 

　　B. strict 

　　C. independent 

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