第三节：完形填空（共20小题；每小题1.5分，满分30分）
阅读下面短文，从短文后各题所给的四个选项（A、B、C和D）中，选出可以填入空白处的最佳选项，并在答题卡上将该项涂黑。
In the past, man didn’t have to think about the protection of his environment. There were few people on the earth, and natural resources seemed to be  21.
Today things are  22  , and the world has become too  23 . We’re using up our natural resources too quickly, and at the same time we are   24 our environment with dangerous chemicals. If we continue to do this, human life on the earth   25   survive.
Everyone 26  today that if too many fish are taken from the sea, there will soon be none left. Yet, with modern fishing  27 , more and more fish are caught. We know that if too many trees are cut down, 28  will disappear and nothing will grow on the land. Yet, we   29 to use bigger and more powerful machines to   30  more and more trees.
We know that if rivers are polluted with waste products from factories, we’ll die. 31 , in most countries wastes are   32  put into rivers or into the sea, and there are   33 laws to stop this.
We know, too, that if the  34  of the world continues to rise at the present rate, in a few years there will not be enough   35  . What can we do to solve these problems? 
If we eat more vegetables and less   36  there will be more food available for every one. Land that is used to grow crops   37 five times more people than land where animals are kept. Our natural resources will  38  longer if we learn to recycle them.
The world population will not rise so quickly if people use modern methods of birth 39   .
Finally, if we educate people to think about the problems, we shall have a better and cleaner  40  in the future.
21.   A. beautiful    B. unlimited      C. rare            D. valuable
22.   A. common    B.the same       C. changeable     D. different
23.   A. crowded     B. small         C. dirty           D. busy
24.   A. protecting   B. saving         C. polluting       D. fighting
25.   A. may not     B. will not        C. shall not       D. could not
26.   A. wonders     B. realizes        C. considers      D. discovers
27.   A. poles        B. boats         C. methods        D. ideas
28.   A. mountains    B. seas          C. trees          D. forests
29.   A. continue     B. have           C. ought         D. go on
30.   A. cut away     B. cut off         C. cut up         D. cut down
31.   A. Thus       B. However       C. Generally speaking D. Therefore
32.   A. still         B. even          C. also           D. certainly
33.   A. too many    B. a few          C. some          D. few
34.   A. production   B. pollution        C. population     D. revolution
35.   A. houses      B. vegetables       C. food          D. lives
36.   A. fruit        B. meat            C. fish          D. grain
37.   A. feeds        B. increases        C. supplies         D. helps
38. A. use          B. stay            C. keep          D. last
39. A. control       B. born            C. plan          D. reward
40. A. nature      B. sea             C. planet         D. forest