There was one thought that air pollution affected only the area immediately around large cities with factories and heavy traffic．At present, we realize that although these are the areas with the worst air pollution, the problem is really 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 distant areas away from any industrial cities．In fact，the very climate of the entire earth may be infected by air pollution．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”—— keeping 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 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．Today we do not know for sure that either of these conditions will happen．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．Driven by economic profits．people ignore the damage on our environment caused by the “advanced civilization”．Maybe the air pollution is the price the human beings have to pay for their development．But is it really worth-while?
66．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 very bad effect on health
D．existed merely in urban and industrial areas
67．As to the greenhouse effect, the author .
A．shares the same view with the scientists．
B．is uncertain of its occurrence
C．rejects it as being ungrounded
D．thinks that it will destroy the world soon
68．The underlined word “offset” in the second paragraph could be replaced by .
A．counter-act B．break from C．get along D．communicate
69．If the world's temperature lowered just a few degrees .
A．it would not do much harm to life on earth
B．it would lead major farming areas to disaster
C．particular things would increase greatly in the atmosphere
D．seas and oceans would flood many cities
70．This passage is primarily about .
A．the greenhouse effect B．the burning of fossil fuels
C．the potential effect of air pollution D．the possibility of a new ice age