题目内容：
回答题。There was one thought that air pollution affected only the area immediately around large citieswith factories and heavy automobile traffic．At present, we realize that although these are the areaswith the worst air pollution, the problem is literally worldwide．On several occasions over the pastdecade, a heavy cloud of air pollution has covered the east of the United States and brought healthwarnings in rural areas away from any major concentration of manufacturing and automobile traffic．
In fact, the very climate of the entire earth may be infected by air pollution．Some scientists considerthat the increasing concentration of carbon dioxide in the air resulting from the burning of fossil fuels(coal and oil)is creating a "greenhouse effect"--conserving heat reflected from the earth and raisingthe world' s average temperature．If this view is correct and the world' s temperature is raised only afew degrees, much of the polar ice cap will melt and cities such as New York, Boston, Miami, andNew Orleans will be in water．
Another view, less widely held, is that increasing particular matter in the atmosphere is bloc-king sunlight and lowering the earth' s temperature--a result that would be equally disastrous．Adrop of just a few degrees could create something close to a new ice age, and would make agriculturedifficult or impossible in many of our top farming areas．Today we do not know for sure that eitherof these conditions will happen ( though one recent government report drafted by experts in the fieldconcluded that the greenhouse effect is very possible)．Perhaps, if we are lucky enough, the twotendencies will offset each other and the world' s temperature will stay about the same as it is now．
Driven by economic profit, people neglect the damage on our environment caused by the "advancedcivilization"．Maybe the air pollution is the price the human beings have to pay for their develop-ment．But is it really worthwhile?
As pointed 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 damaged effect on health
D．existed merely in urban and industries areas
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