单选题 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 traffic. In fact, the very climate of the entire earth may be infected by air pollution. Some scientists 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 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 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.

单选题 As pointed out at the beginning of the passage, people used to think that air pollution ______.

【正确答案】 D

【答案解析】[解析] 第一段第一句：“There was one thought that air pollution affected only the area immediately around large cities with factories and heavy automobile traffic. ”
事实细节题。根据定位句可知，过去人们以为空气污染仅影响拥有工厂和拥堵交通的大城市及其周围区域。故选D。

单选题 As to the greenhouse effect, the author ______.

【正确答案】 B

【答案解析】[解析] 第一段最后一句：“If this view is correct and the world"s temperature...”第二段第三句：“Today we do not know for sure that either of these conditions will happen...”
推理判断题。根据定位句可知，作者还不能确定温室效应等一些情况是否会发生。故选B。

单选题 The word "offset" in the second paragraph could be replaced by ______.

【正确答案】 B

【答案解析】[解析] 最后一段最后一句：“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. ”
推理判断题。根据定位句可知，如果我们足够幸运，这两种趋势会相互抵消，那么全球的气温就会保持现状。所以只有冷暖相互弥补，才能出现“气温和现在一样正常”。slip into滑进；make up for弥补；set up建立，创造；catch up with追上，赶上。故选B。

单选题 It can be concluded that ______.

A、 raising the world"s temperature only a few degrees would not do much harm to life on earth
B、 lowering the world"s temperature merely a few degrees would lead many major farming areas to disaster
C、 almost no temperature variations have occurred over the past decade
D、 the world"s temperature will remain constant in the years to come

【正确答案】 B

【答案解析】[解析] 全文。
推理判断题。根据第二段第二句“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. ”可知，仅降温几度就会导致农业生产变得困难甚至无法生产，这对农业来说是巨大的灾难，所以B正确；根据第一段最后一句“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. ”可知，温室效应可能导致南极的冰雪融化，将城市淹没，所以A不正确；根据常识，近几十年气温在逐渐升高，所以C不正确：文中提到全球气温的变化受空气污染的影响，所以只要污染存在，气温就不可能恒定不变，D说得太绝对了。故选B。