单项选择题
The world is not only hungry, it will also be thirsty for water. About 97% of water on the earth is sea water, or salt water. Man can only drink or use the other 3% of the fresh water which comes from rivers, lakes and underground. On the other hand, because the number of people in the world is becoming larger and larger. So water shortage (短缺) becomes a big problem.
How can we work out the problem? Scientists have found some ways to turn salt water into frosh water. One important way is to boil salt water with high heat. In this way, a lot of fresh water can be made quickly.
But heating is not the only way to get fresh water. Other ways are tried, for example, digging deep wells, keeping rain water and stopping rivers from becoming dirty.
Which way is the best? To get the most fresh water with the least money is the best.
A.
is needed by more and more people
B.
can be used in many ways
C.
can be found in many factories
D.
only comes from the rain
