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  3. It was once thought that air pollution affected only the area immediately around large cities with factories and heavy automobil

It was once thought that air pollution affected only the area immediately around large cities with factories and heavy automobil

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问题     It was once thought that air pollution affected only the area immediately around large cities with factories and heavy automobile traffic. Today, we know 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 entire eastern half of the United States and led to health warnings even in rural areas away from any major concentration of manufacturing and automobile traffic. In fact, the very climate of the entire earth may be affected by air pollution.
Some scientists feel 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"—holding in 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 under water.
    Another view, less widely held, is that increasing particulate 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 new ice age and would make agriculture difficult or impossible in many of our top farming areas.
    At present we do not know for sure that either of these conditions will happen, though one recent government report prepared by experts in the field concluded that the greenhouse effect is very likely. Perhaps, if we are very lucky, the two tendencies will offset each other and the world’s temperature will stay about the same as it is now.  
The word "offset" (Line 3, Para. 4) means________.
选项 A、slip into
B、catch up with
C、set up
D、make up for
答案D
解析 由题干中“offset”可以定位到文中最后一段最后一句话。该句提到如果这两种趋势能够中和一下，那全球的气温会跟现在差不多，由此可推断出“offset”意为“中和，抵消”。
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