第4部分：阅读理解（第31～45题，每题3分，共45分）
下面有3篇短文，每篇幅短文后有5道题，每题材后面有4个选项。请仔细阅读短文并根据短文回答其后面的问题，从4个选项中选择一个最佳答案涂在答题卡相应的位置上。
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 earth's 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 （冰山） and glaciers （冰川）. 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 sea or runs into underground storage tanks. But it is possible to pipe water that has been used to a purifying （净化） plant. There it can be filtered （过滤） and treated with chemicals so that it can be used again just as if it were fresh from a spring.
But even if every large city purified and reused its water, we still would not have enough. Where could we turn next? To the oceans! All we'd have to do to make use of the vast amount of sea-water is - remove the salt. This salt-removing process is already in use in many parts of the world.
So if we take all these steps, we'll be in no danger of drying up!
