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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.
   
选项
答案G
解析 D为首段，其末句引用了一个study得出的结果，可推测后一段应该会详细介绍该研究，比如方法、过程和结果等。G恰恰给出了该项研究的相关信息和数据，并且G首句中的so precise a finding和前文中具体的数字(an average of five months)对应，因此G应紧跟在D之后。
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