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  3. [A] One of the reasons making air cleaner can have so immediate an effect is that even a little dirt can do a lot of damage. A r

[A] One of the reasons making air cleaner can have so immediate an effect is that even a little dirt can do a lot of damage. A r

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问题    [A] One of the reasons making air cleaner can have so immediate an effect is that even a little dirt can do a lot of damage. A reduction of just 10 micrograms of pollution per cubic meter of air—a degree of improvement many of the surveyed cities were able to attain during the two-decade-plus period-could extend human life-spans a full nine months. How small is 10 micrograms per cubic meter? Consider that simply by living with a cigarette smoker, you’re exposed to a daily dose of 20 to 30.
   [B] As with so many other things, President Barack Obama’s coming into power has people hoping that these kinds of questions will be more aggressively addressed than they were over the past eight years. Even during the most heated days of the fall campaign, neither candidate went so far as to promise longer life in exchange for a vote. But a smart environmental policy could deliver just that.
   [C] The benefits of cleaner air may even be felt in towns whose skies weren’t that dirty to begin with. Those that began with the very lowest levels still saw health benefits from small improvements. The evidence isn’t yet there to determine whether those benefits would continue growing until the fine-particle pollution got down to zero; one of the cities closest to that, Albuquerque, New Mexico, still hovers around 5 micrograms per cubic meter. But at this point, it doesn’t seem that the benefits decrease. "If it continues to follow what we’ve observed, it appears that there are health benefits down to very low levels of exposure," says Dr. Pope, the study’s lead author.
   [D] Nobody pretends that polluted air isn’t terrible for your health. Clean up the skies over any dirty city, and the people who live there will all but certainly become healthier. That, at least, has been popular wisdom, but until now, no one had ever put it to a statistical test. Now someone has, and the results are striking: according to a study just published, when local governments decide to remove the smog, local residents actually live an average of five months longer.
   [E] Pittsburgh, Pennsylvania, for example, is one city in the survey that was at the 30-microgram level before the decline of the steel industry in the 1980s drove the dirt out of the skies—even as it drove jobs out of town. Pittsburgh was one of the biggest winners in the new study, with residents gaining roughly 10 months in life expectancy over what they had when the mills were still churning.
   [F] The next step for both researchers and policymakers is determining which sources of dirt—power plants, motor vehicles, other industrial polluters—make the biggest contributions to particle levels and thus should be most aggressively targeted. "In a difficult economic situation," asks Dr. Douglas Dockery, "where can we spend the dollars that would have the most benefit?"
   [G] In order to reach so precise a finding, the study’s authors had to do some exhaustive number-crunching, surveying pollution rates and longevity in 51 cities across the U.S. over a 21-year period from 1979 to 2000. Overall, they found that lifespan in all of the areas increased by an average of nearly three years—from 74 to 77—as a result of a host of factors, most notably reduced smoking and improved income. But 15% of the change was attributable to cleaner air.
   
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答案E
解析 E和C都以城市的情况举例，说明净化空气对人体健康产生的有益影响。而E的首句恰好谈到“参与调查的匹兹堡曾经空气中的污染物含量为30微克／立方米(at the 30-microgram level)”，这正好衔接了A最后一句提到的you’re exposed to a daily dose of 20 to 30，即：上段提到减少10毫克／立方米如此小的量，就有延长9个月寿命之效，末句讲到可以想象一下减少20-30毫克／立方米的量会有多好的延寿效果；E紧接着结合与上述污染程度相当的匹兹堡改善空气后的效果，对上段观点进行说明。可见E应紧接在A之后。
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