It was once thought that air pollution affected only the area immediately around large cities with factories and/or heavy traffic. Today, we know that although these are the areas with the worst air pollution, the problem is actually worldwide. On several occasions (情形) over the past ten years, 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 industrial and traffic center. In fact, the very climate (气候) of the entire earth may be affected by air pollution. Some scientists feel that the increase of carbon dioxide in the air which comes from the burning of fossil fuels (化石燃料) , such as 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 (颗粒物) in the atmosphere is blocking sunlight and lowering the earth’s temperature ─ a result that would be equally full of danger. 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 (趋势) will offset each other and the world’s temperature will stay about the same as it is now.

64. People used to think that air pollution ______.

       A. affected the entire eastern half of the United States

       B. had damaging effects on health

       C. existed only in urban and industrial areas

       D. caused widespread damage in the countryside

65. As far as the greenhouse effect is concerned, the author ______.

       A. is uncertain of its happening

       B. thinks that it will destroy the world soon

       C. refuses it as being ungrounded

       D. shares the same view with the scientists

66. The underlined word “offset” probably mean ______ in Chinese.

       A. 强化                     B. 抵消                  C. 并存                          D. 补偿