题目内容：
根据下面材料，回答题。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.____46____, because some of it is in the form of icebergs and glaciers. Even worse, some of it has been polluted.
At the moment, his 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. One of the useful steps we can take is to stop unlimited use of water.____47____, however, would have a bad effect on agriculture and industry.
In addition to stopping wasting our precious water, one more useful step we should take is to develop ways of reusing it.____48____, 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 plant. There it can be filtered and treated with chemicals ____49____ .
____50____, 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 !
第46题__________ A．A limited water supply
B．so that it can be used again just as if it were fresh from a spring
C．It is possible to purify large amounts of sea water
D．But even if every large city purified and reused its water
E．And we cannot even use all of that
F．Experiments have already been done in this field
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