Sugar is so much a part of our modern life that we only really think about it when, for some ___1___ , we cannot obtain it. It has been known to man for at least 3,000 years, but has ___2___ into common use only in ___3___times. Until quite recently it was considered as a medicine and as a luxury for the very rich only.
　　Sugar is, then, ___4 ___to our civilization. But what___5___ is it? Of course, most of us recognize sugar immediately as the sweet material which 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 under ground). But there are in fact many types of sugar, and the chemist recognizes hundreds of different ___6___ , each coming from a different source.
　　About 90% of the sugar is produced as food. Only 10% is used in industry for ___7___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 ___8___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 acre. In this way they have the advantage over potatoes which give only 4, 000, 000, while the___9___ for wheat and beans is 2 ,000,000 each. So three acres of land growing wheat, beans and potatoes give only ___10__more energy than one acre of sugar.

　　A. slightly B. intention C. reason D. modern
　　E. strongly F. figure G. come H. significant
　　I. exactly J. increase K. proposals L. turn
　　M. purposes N. varieties O. serious

　Key：I. C 2. G 3. D 4. H 5. I 6. N 7. M 8. J 9. F 10. A