Section BDirections: There are 2 passages in this section. Each passage is followed by som
Section B
Directions: There are 2 passages in this section. Each passage is followed by some questions or unfinished statements. For each of them there are four choices marked A, B, C and D. You should decide on the best choice.
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.
Of all the reserves of fresh water in the world, about three-fourths is ______.
A．consumed by glaciers and polar ice
B．in solid form
C．out of the reach of present technology
D．converted into sea water annually
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