单项选择题

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, (76) salt water. Man can only drink and use the other 3% of the fresh water that comes from rivers, lakes, underground, and other (77) 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.
However, as things stand today, this small amount of fresh water, (78) is constantly being replaced by rainfall, is still enough for us. But our need (79) water is increasing rapidly—almost day by day. Only if we take steps to deal with this problem now (80) a severe worldwide water shortage later on. A limited water supply would have a bad effect on agriculture and industry. Let me give you just one small example of (81) necessary water is to industry. Do you know that (82) a single ton of steel, it takes about 91,000 liters of water
We all have to learn how to stop (83) our precious water. One of the first steps we should take (84) to develop ways of reusing it. Experiments have already been done in this field, but only on a small scale. The systems that have been worked out resemble (85) used in spacecraft.

80（）