听力原文： Sugar is so much a part of our modern life that we only really think about it when, for some reason, we cannot obtain it. It has been known to man for at least 3,000 years, but has come into common use only in modern times. Until quite recently it was considered as a medicine and as a luxury for the very rich only. Sugar is, then, very important to our civilization. But what exactly is it? Of course, most of us recognize sugar immediately as the sweet material that we put in coffee or cakes. This common form. of sugar is derived from two plants: the sugar cane (a type of grass which grows to a height of twenty feet) and the sugar Beet (which grows underground). But there are in fact many types of sugar, and the chemist recognizes hundreds of different varieties, each coming from a different source. About 90% of the sugar is produced as food. Only 10% is used in industry for purposes other than food production. Yet sugar has great possibilities for use as the basis of chemicals. It can even be used for making plastics. In the future these potential uses will certainly be developed more than in the past. There are many reasons why we should increase the production of sugar. Most important is that it is one of the most highly concentrated of energy foods. Thus sugar cane and beet produce an average of 7,000,000 calories per a-ere. In this way they have the advantage over potatoes, which give only 4,000,000, while figure for wheat and beans, is 2,000,000 each. So three acres of land growing wheat and potatoes give only slightly more energy than one acre of sugar. What is the percentage of the sugar used as food?