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  3. The long, wet summer here in the northeastern U.S. notwithstanding, there’s a world shortage of pure, fresh water. As demand for

The long, wet summer here in the northeastern U.S. notwithstanding, there’s a world shortage of pure, fresh water. As demand for

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问题     The long, wet summer here in the northeastern U.S. notwithstanding, there’s a world shortage of pure, fresh water. As demand for water hits the limits of finite supply, potential conflicts are brewing between nations that share transboundary freshwater reserves.
    Many people ask why we cannot simply take it from the sea, using our sophisticated technology of desalinization. But a good water supply must be hygienically safe and pleasant tasting and water containing salt would corrode machinery used in manufacturing in addition to producing chemical impurities. Since more than 95% of our water sits in the salty seas, man is left to face the reality that most water on the surface of the earth is not available for us.
    One very feasible way of sustaining our supply of freshwater is to protect the ecology of our mountains. Mountains and water go together, a fact to which Secretary General Kofi Annan has drawn attention more than once. From 30% to 60% of downstream fresh water in humid areas and up to 95% in arid and semi-arid environments are supplied by mountains. Without interference nature has its own way of purifying water—even though chlorination and filtration are still necessary as a precaution. In a mountainous area, aeration, due to turbulent flow and waterfalls, causes an exchange of gases between the atmosphere and the water. Agriculture, industry, hydroelectric generators and homes that need water to drink and for domestic use depend on these resources and, thus, we must protect mountainous areas as a means of survival.
Streams would purify themselves if not for ______.
选项 A、human beings
B、nature
C、chlorination
D、mountains
答案A
解析 从第3段可知，在没有干预的情况下，大自然用自己的方式净化水源。因此A项为正确答案。
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