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  3. It was once thought that air pollution affected only the area immediately around large cities with factories or heavy automobile

It was once thought that air pollution affected only the area immediately around large cities with factories or heavy automobile

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问题    It was once thought that air pollution affected only 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 literally worldwide. On several occasions over the past decade, a heavy cloud of air pollution had covered the entire eastern half of the United States and led to health warnings even in rural areas away from any major concentration of manufacturing and automobile traffic. In fact, the very climate of the entire earth may be affected by air pollution. Some scientists feel that the increasing concentration of carbon dioxide in the air resulting from the burning of fossil fuels (coal and oil) is creating a "greenhouse 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 cap will melt 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 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 a 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 the field concluded that the greenhouse effect is very likely). Perhaps, if we are very lucky, the two tendencies will offset each other and the world’s temperature will stay about the same as it is now.
It can be inferred from the passage that _____.
选项 A、raising the world’s temperature only a few degrees would not do much harm to life on the earth
B、lowering the world’s temperature merely a few degrees would lead many major farming areas to disaster
C、almost no temperature variations have occurred over the past decade
D、the world’s temperature will remain constant in the years to come
答案B
解析 文中第二段第二句明确指出，如果地球温度降低几度，会给农作物及农业生产带来灾难。选项A、C在文中均无线索，选项D只是作者有假设条件的推测，并不一定是客观情况。
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