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  3. One way of preventing floods may be by making rain ! This may sound rather strange. As we know, heavy rain is the cause of flood

One way of preventing floods may be by making rain ! This may sound rather strange. As we know, heavy rain is the cause of flood

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问题     One way of preventing floods may be by making rain ! This may sound rather strange. As we know, heavy rain is the cause of floods, and in Malaysia most of the floods are caused by the annual monsoon (季风), which brings heavy rain to the east coast. If, however, the rain could be made to fall in the China Sea, there would be no more floods since the rain clouds would not reach the east coast.
    Is it possible to make clouds give up their rain.? Yes, it is, and in 1973 the National Institute of Scientific and Industrial Research (NISIR) suggested that it would be tried. The institute knew of course that it is not possible to make all clouds give up all their rain. It calculated, however, that if the frequency of rainfall could be reduced by 10%, a/id the amount of rain that fell each time could be reduced by 15%, the total amount of flood damage would be reduced by 40%—50%.
    As you probably know, clouds are caused by air containing water-vapor. The air rises and, since water-vapor becomes water when it cools, very tiny drops of water are formed. Since these drops are, very small, they float in the air, and form a cloud. When the cloud rises, however, the drops become colder. Because of this, they join together and become big drops. The big drops are no longer able to float be cause of their size, and they fall. As they fall, they pick up more drops and become bigger. This is the cause of rain.
What does "it" of "since water-vapor becomes water when it cools" in Para. 3. refer to?
选项 A、Water-vapor.
B、Cloud.
C、Water.
D、Air.
答案D
解析 要弄清楚该句中的指代关系，就必须正确理解文章第三段的内容——雨是如何形成的。云是由含有水蒸气的空气形成的。空气不断上升、冷却，水蒸气变成小水滴。当水滴越变越大，再也无法飘浮在空中时就变成了雨。由此可知句中的it指的是空气。
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