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 seawater,21 salt water. Man can only drink and use the other 3% of the fresh water that comes from rivers, lakes, underground, and other22 .
And we can't 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.
However, as things stand today, this small amount of fresh water, which is constantly being replaced by rainfall; is still enough for us. But our need（3）water is increasing rapidly－almost day by day. We all have to learn how to stop wasting our previous water. One of the first steps we should24 is to develop ways of reusing it.
Experiments have already been done in this （5）, but only on a small scale. The systems that have been worked out resemble those used in spacecraft.
3、_______

A.take

B.or

C.sources

D.dfor

E.ecase