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  3. Global reserves of fresh water add up to more than 37 million cubic kilometers, enough to fill the Mediterranean 10 times over.

Global reserves of fresh water add up to more than 37 million cubic kilometers, enough to fill the Mediterranean 10 times over.

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问题     Global reserves of fresh water add up to more than 37 million cubic kilometers, enough to fill the Mediterranean 10 times over. More than three-fourths of this water is bound up in glaciers(冰川)and polar ice, however, where it is largely beyond the reach of present technology. Almost all the rest consists of water in underground aquifers(蓄水层), which are not yet exploited intensively. The main sources of supply -- the waters of lakes and rivers and the water vapor in the atmosphere -- make up less than I percent of the total.
    The ultimate source of fresh water is the continuous distillation(蒸镏)of the oceans by solar radiation. The annual evaporation of water(including transpiration by plants)is roughly 500,000 cubic kilometers, of which 430,000 comes from the oceans and the remaining 70,000 from waters on the continents. Because the amount of water vapor in the atmosphere is essentially constant, the same amount of water must fall back to the surface as rain and snow. It is of vital importance to earthly life that a disproportionate share of this precipitation falls on land. Whereas the continents lose 70,000 cubic kilometers of water to evaporation, they receive 110,000 from precipitation, so that the net effect of the water cycle is to transfer some 40,000 cubic kilometers of fresh water each year from the oceans to the continents,
    Although the net continental influx(汇集)is 40,000 cubic kilometers per year, not all of it is available for man’s use. Much is lost through floods or is held in the soil or in swamps. The maximum that might reasonably be applied to human purposes is about 14,000 cubic kilometers per year, which is the base flow, or stable runoff excluding flood waters, of all the world’s rivers and streams and of those isolated underground aquifers that discharge directly through evaporation. Of this volume about 5,000 cubic kilometers flow in regions that are uninhabited and are likely to remain so because they are climatically unsuited to human settlement. Hence the effective world water resource, from which all needs will have to be met for some years to come, is about 9,000 cubic kilometers per year.
The word "precipitation"(Line 6, Para. 2)probably means ______.
选项 A、the evaporation of water from the land
B、the water reserves on land
C、the unevenly-distributed rain and snow falls
D、water falling in the form of rain and snow
答案D
解析 可以从该词所在的上下文来推测。该词所在的上一句说：因为水蒸气基本是以雨和雪的形式落到地面的，接下来该词所在的句子又说：地球上的生命大量需要这种落到地上的水，可知precipitation相当于rain and snow，所以选D)，意思是“(雨、雪等的)降落”。A)“从地面蒸发的水蒸气”；B)“保存在地面的水”；C)“不均衡分布的降雨和降雪”。
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