As to the greenhouse effect, the author________根据下列材料，请回答下面的题目：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 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 blockingsunlight and lowering the earth’s temperature--a result that would be equally disastrous. A drop ofjust a few degrees could create something close to a new ice age, and would make agriculture difficultor impossible in many of our top farming areas. Today we do not know for sure that either of theseconditions will happen (though one recent government report drafted by experts in the field concludedthat the greenhouse effect is very possible). Perhaps, if we are lucky enough, the two tendencies willoffset each other and the world’s temperature will stay about the same as it is now. Driven by eco-nomic profit, people neglect 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 worthwhile?As pointed out at the beginning of the passage, people used to think that air pollution________

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