Man depends on water for many things. He needs it for drinking. He needs it for growing food. He needs it for keeping himself clean and free from illness. He needs more and more water for industry. People often forget how necessary water is for industry. Imagine, for example, the immense quantities of water used by the great cooling towers of steel works. The demand for water is growing every day. It is closely connected with the increasing population and with the pressing problem of providing enough food. But the world has not yet found ways of storing enough water to satisfy all these important needs, not even in west countries like England. Great efforts are being made today to store water, particularly in hot countries where the rainfall is small. Three-quarters of the world is covered with water. But only three percent of this water is fresh. All the rest is salt, and fills the oceans and the great inland seas. It is the salt that makes sea water useless to man. If you take the salt away the water can be used for drinking and for watering plants. In 1962 President John Kennedy of the United States said, 'When man discovers how to mm salt water into fresh water cheaply, he will have made a much more important scientific advance than when he first landed on the moon'. Human beings need water for【46】,【47】and【48】and【49】. Main idea: The【50】