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  3. Ozone in the stratosphere blocks deadly ultraviolet rays from the sun, but chlorofluorocarbons (CFCs) in aerosols and other prod

Ozone in the stratosphere blocks deadly ultraviolet rays from the sun, but chlorofluorocarbons (CFCs) in aerosols and other prod

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问题 Ozone in the stratosphere blocks deadly ultraviolet rays from the sun, but chlorofluorocarbons (CFCs) in aerosols and other products have thinned this protective layer. Evidence of this is the ozone hole that forms over the South Pole every Antarctic spring as temperatures drop below -78°C, the temperature at which ozone depletion occurs. Measurements of the ozone hole taken at various times this spring show that, compared with the same times the previous year, its area diminished by four million square kilometers. Nevertheless, scientists have not concluded that the ozone layer is recovering.
Which of the following would, if true, provide the strongest reason for the scientists’ reaction to the measurements?
选项 A、The ozone hole has steadily grown in size every year for the past decade except this year.
B、The length of time that the ozone hole persists fluctuates from year to year.
C、As a result of international treaties, CFCs have been completely banned for several years.
D、Weather patterns allowed unusual amounts of warm air to mix into the polar regions this year.
E、Human-made CFCs retain their ability to destroy ozone molecules for seventy-five to one hundred years.
答案D
解析 This question requires us to identify which of the answer choices would provide the strongest reason
to support the scientists’ skepticism about whether the ozone layer is recovering.
This skepticism exists despite the fact that measurements of the ozone hole taken at various times during the Antarctic spring have shown that the hole has diminished significantly from its size at the same times the previous year.
One major reason to be skeptical would be if there were some factor other than a recovery of the ozone layer that could reasonably account for the diminished size of the hole.
A    This does not give much reason to be skeptical that the ozone layer is recovering. Certainly one would not want to be too hasty in declaring a recovery after noting a trend of growth in the ozone hole’s size. Nevertheless, something must account for this year’s divergence from the trend. It seems reasonable to conclude, barring the discovery of some other factor that would explain the change, that a recovery in the ozone layer could be a factor in this year’s divergence.
B    Each of the measurements was smaller than at the same time in the previous year. This fact would seem to indicate that fluctuations in the length of time the ozone hole persists do not justify the scientists’ skepticism.
C    The fact that CFCs—which led to the thinning of the ozone layer—have been banned counts against the scientists’ skepticism rather than supports it.
D    Correct. The hole forms when the temperature drops below -78°C during the Antarctic spring. If much of the area where the hole appears was significantly warmer than the previous year—perhaps above -78°C—there exists a reason other than a recovery in the ozone layer that explains the smaller size of this year’s hole in the ozone. This would support the scientists’ skepticism.
E    Even if CFCs retain their ability to destroy ozone molecules for many decades, something must account for the decrease in the size of the hole. Nothing in the passage indicates whether the amount of CFCs in the atmosphere has been increasing or decreasing. If it has been decreasing for a long time, then this fact is compatible with a belief that the ozone layer is recovering.
The correct answer is D.
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