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阅读理解专项训练四 下面有3篇短文，每篇短文后有5道题，每道题后面有4个选项。请根据文章的内容，从每题所给的4个选项中选择1个最佳答案。 第一篇 A Thirsty World The world is not only hungry; it is also thirsty for water. This may seem strange to you, since nearly 75% of the earths surface is covered with water. But about 97% of this huge amount is sea-water, or salt water. Man can only drink and use the other 3% - the fresh water that comes from rivers, lakes, underground, and other sources. And we cannot even use all of that, because some of it is in the form of icebergs and glaciers . Even worse, some of it has been polluted. At the moment, this small amount of fresh water is still enough for us. However, our need for water is increasing rapidly. Only if we take steps to deal with this problem now can we avoid a severe worldwide water shortage later on. A limited water supply would have a bad effect on agriculture and industry. In addition to stopping wasting our precious water, one useful step we should take is to develop ways of reusing it. Experiments have already been done in this field, but only on a small scale. Today, in most large cities, water is used only once and it eventually returns to the see or runs into underground storage tanks. But it is possible to pipe water that has been to a purifying plant. There it can be filtered and treated with chemicals so that it can be used again just as if it ware fresh from a spring. But even if every large city purified and reused its water, we still would not have where could we turn next? To the oceans! All we d have to do to make use of vast amount of sea-water is - remove the salt. This salt-removing process is already use in many parts of the world. | 
