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根据短文内容，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 ponution has covered the east of the United States and brought health warnings 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 pohufion．Some scienfists 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"--conserving 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 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. 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 by experts in the field concluded that the greenhouse effect is very 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 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 damaging effect on health
D．existed merely in urban and industries areas
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