It was once thought that air pollution only affected the area immediately around large cities with factories or heavy automobile traffic. Today we know that although these are the areas with the worst air pollution, the problem is actually worldwide. On several occasions over the past ten years, a heavy cloud of air pollution has covered the entire eastern half of the United States and led to health warning even in rural areas away from any major concentration of big factories or automobile. In fact, the very climate of the entire earth may be affected by air pollution. Some scientist feel that the increasing concentration of carbon dioxide in the air resulting from the burning of coal and oil is creating a "green house" effect--holding in heat reflected from the earth and raising the world's average temperature. If this view is correct and the world's temperature is raised only a few degrees, much of the polar ice will disappear and cities, such as New York, Boston, Miami, and New Orleans will be under water.
Another view, less widely held, is that increasing particulate matter (颗粒物质) in the atmosphere is blocking sunlight and lowering the earth's temperature--a result that would be equally disastrous. A drop of just a few degrees could create something close to new ice age and would make agriculture difficult or impossible in many of our top farming areas. At present we do not know for sure that either of these conditions will happen (though one recent government report prepared by experts in this field concluded that the green house effect is very likely). Perhaps, if we are lucky enough, the two trends will offset each other and the world's temperature will stay about the same as it is now.
72. As far as the green house effect is concerned, the author_________.
A. shares the same view with the scientist
B. is not sure that it will happen
C. thinks it impossible because there is no strong evidence
D. thinks it will destroy the world soon
73. According to what we read in the passage, _________.
A. raising the world's temperature only a few degrees would do no harm to life on the earth
B. lowering the world's temperatures only a few degrees would lead many major farming areas to disaster
C. the temperature in the countryside will remain permanent in the years to come
D. the particulate matter in the atmosphere has lowered the world's average temperature
74. The word "offset" in the last sentence means_________.
A. strengthen      B. worsen     C. balance      D. support
75. This passage is mainly about_________.
A. the pollution caused by human beings        B. the green house effect
C. the potential effect of air pollution            D. the possibility of a new ice age

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