题目内容：
根据以下内容回答题,There was one thought that air pollution affected only the area immediately around large cities with factories and heavy automobile traffic．At present，we realize 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 has covered the east of the United States and brought health warnings in rural areas away from any major concentration of manufacturing and automobile tmmc．In fact，the very climate of the entire earth may be infected by air poilu-tion．Some scientists consider 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”——con-serving heat refleeted from the earth and raising the world’S average temperature．If this view is correct and the worid’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 in water．
Another view．1ess widely held，is that increasing particular matter in the atmosphere is blocking sunlight and lowering the earth’s temperature--a result that would be equally disas-trous．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 fanning areas．Today we do not know for sure that either of these conditions will happen(though one recent government report drafted bv experts in the field concluded that the greenhouse effect is#cry possible)．Perhaps，if we are lucky enough，the two tendencies will offset each other and the world’s temperature will stay about the same as it is now．
As Dointed out at the beginning of the passage，people used to think that air pollution__________ ． A．caused widespread damage in the countryside
B．affected the entire eastern half of the United States
C．had a damaging effect on health
D．existed merely in urban and industries areas
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