听力原文： The world is not only hungry, but thirsty for water. That 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%-tile fresh water 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.
However, this small amount of fresh water is still enough for us. But our need for water is increasing rapidly. Only if we take steps to deal with the problem now can we avoid a severe world-wide water shortage later on.
One of the first steps is to develop ways of reusing it. Today water is used only once and then discharged into a sewer system, and it eventually returns to the sea or runs into underground storage tanks. But it is possible lo pipe used water to a purifying plant. There it can be filleted and treated with chemicals so that it can be used again.
But even if every large city purified and reused its water, we would still not have enough.
All we'd have to do to make use of the sea water in, the world is to remove the salt. If we take these steps we'll be in no danger of drying up!
How much of the earth’s water can man really use？

A. He can use nearly 75% of it.
B. He can use about 97% of it.
C. He can use exactly 3% of it.
D. He can use less than 3% of it.