The world is not only hungry but also thirsty for water. This may seem strange to you, since nearly 70% of the earth's surface is covered with water. But about 97% of that is sea water or salt water. Man can only drink and use the other 3% of fresh water from rivers, lakes, underground and so on. And we can not even use all of that because some of it has been made dirty. Earth Day is April 22. But on all other days, we must also remember how important water is. The water that we use is the most important natural resource on the earth. Today we face serious water problems. One of them is water pollution. All kinds of things from cars, factories, farms and homes make our rivers, lakes and oceans dirty. Polluted water is very bad for people to drink. And dirty water is bad for fish, too. How do cars and factories make our water dirty? First they pollute the air. Then when it rains, the rain water makes our drinking water dirty. Dirty rain, called acid rain is also bad for plants, animals and buildings. Scientists say that in 30 years, more than half of the people in the world won't have enough clean water. We have to learn how to stop wasting water. One of the first steps is to develop ways of reusing it. Today in most large cities, water is used only once and then sent out into a sewer system  . From there it returns to the sea or goes underground. But even though every large city reused its water, we would still not have enough. So we should make use of sea water by removing the salt in it. If we can take these steps, fresh water won't be used up. The world is thirsty for water because   _  .
we don't have enough fresh water a lot of water goes into sewer systems only 70% of the earth's surface is water sea water is widely used rather than fresh water