A、By filtering sea water. B、By treating sea water with chemicals. C、By taking salt out of sea water. D、By drying up sea water. C

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. People 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.
   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 useful step we should take is to develop ways of reusing it.
   Today, in most large cities, water is used only once. But it is possible to pipe water that has been used to a purifying plant. There it can be filtered 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 to 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!