大学英语四六级考试阅读提高的练习（38）

　　We cannot obtain it. It has been known to man for at least 3,000 years, but has 12 into common use only in 13 times. Until quite recently it was considered as a medicine and as a luxury for the very rich only.

　　Sugar is, then, 14 to our civilization. But what 15 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 and the sugar beet . But there are in fact many types of sugar, and the chemist recognizes hundreds of different 16 , each coming from a different source.

　　About 90% of the sugar is produced as food. Only 10% is used in industry for 17 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 18 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 19 for wheat and beans is 2 ,000,000 each. So three acres of land growing wheat, beans and potatoes give only 20 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

　　II. C 12. G 13. D 14. H 15. I 16. N 17. M 18. J 19. F 20. A