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，掌握其大意，然后从各题所给的四个选项（A、B、C和D）中，选出最佳选项。
The climate will be warm and __21___ with prolonged droughts in most of the world’s grain growing ___22__. Because of the increase in temperature, the ice cap will begin to melt and sea level is predicted to __23___ about ten meters, causing flood in many areas.
In the atmosphere, a concentration of carbon dioxide acts like the _24__ in the roof of a greenhouse. It allows the sun’s rays to pass _25___ easily on the way to the earth’s _26__, but prevents heat _27__ by the earth from escaping back into space. __28__, as the level of atmospheric carbon dioxide rises,because of the industrial burning of fossil fuels, this greenhouse effect could __29__ the temperature of the earth. The burning of fossil fuels __30__ carbon dioxide. Carbon dioxide accumulates(积聚) in the atmosphere, and __31_ the greenhouse effect, the temperature of the earth’s surface could rise. __32__ this warming up took place, the polar ice caps would begin to melt,__33__raising sea level. The __34__ influence that seems to be cooling the earth is the variable behavior of the sun. As the sun rotates(旋转) on its orbit, it presents hotter or colder faces to the earth, and this seems to have a considerable effect __35__ the distribution of the earth’s atmospheric pressure and on the wind circulation. The heat __36__ of the sun also varied, the latest trend being __37__, the heat output appears to be decreasing and the indications are that the earth’s __38_ is entering a cooler cycle. Scientific models that take __39__ account this __40__ behavior of the sun are also predicting that the world should be entering a new Ice Age. 
21. A. wet           B. rainy               C. dry                 D. icy
22. A. farms               B. regions              C. towns               D. villages
23. A. rise             B. appear                 C. move              D. raise
24. A. glass            B. shylight(天窗)     C. chimney       D. attic(顶楼)
25. A. by              B. over                    C. in                    D. through
26. A. ground        B. bottom                C. floor              D. surface
27. A. taken in      B. sent out                    C. admitted to     D. taken out
28. A. However     B. Yet                    C. Afterwards        D. Therefore
29. A. benefit               B. forbid                  C. raise             D. effect
30. A. leads to     B. gives off              C. results from      D. joins hands
31. A. as to           B. as for                         C. without         D. because of
32. A. Whether      B. Before                   C. If                   D. After
33. A. so             B. instead                    C. thus            D. but
34. A. natural      B. unusual              C. former                     D. ordinary
35. A. in                      B. on                         C. over                 D. at
36. A. input           B. activity              C. output           D. movement
37. A. downward        B. upward                    C. forward          D. onward
38. A. pressure    B. surface              C. climate         D. appearance
39. A. from         B. into                   C. in                   D. on
40. A. unbelievable B. dull                         C. variable         D. hidden