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  3. Community cancer clusters are viewed quite differently by citizen activists than by epidemiologists. Environmentalists and conce

Community cancer clusters are viewed quite differently by citizen activists than by epidemiologists. Environmentalists and conce

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问题     Community cancer clusters are viewed quite differently by citizen activists than by epidemiologists. Environmentalists and concerned local residents, for instance, might immediately suspect environmental radiation as the culprit when a high incidence of cancer cases occurs near a nuclear facility. Epidemiologists, in contrast, would be more likely to say that the incidences were "inconclusive" or the result of pure chance. And when a breast cancer survivor, Lorraine Pace, mapped 20 breast cancer cases occurring in her West Islip, Long Island, community,  her rudimentary research efforts were guided more by hope--that a specific environmental agent could be correlated with the cancers than by scientific method.
    When epidemiologists study clusters of cancer cases and other noncontagious conditions such as birth defects or miscarriage, they take several variables into account, such as background rate (the number of people affected in the general population), cluster size, and specificity (any notable characteristics of the individual affected in each case). If a cluster is both large and specific, it is easier for epidemiologists to assign blame. Not only must each variable he considered on its own, but it must also be combined with others. Lung cancer is very common in the general population. Yet when a huge number of cases turned up among World War Ⅱ shipbuilders who had all worked with asbestos, the size of the duster and the fact that the men had had similar occupational asbestos exposures enabled epidemiologists to assign blame to the fibrous mineral.
    Although several known carcinogens have been discovered through these kinds of occupational or medical clusters, only one community cancer cluster has ever been traced to an environmental cause. Health officials often discount a community’s suspicion of a common environmental cause because citizens tend to include cases that were diagnosed before the afflicted individuals moved into the neighborhood. Add to this the problem of cancer’s latency. Unlike an infectious disease such as cholera, which is caused by a recent exposure to food or water contaminated with the cholera bacterium, cancer may have its roots in an exposure that occurred 10 to 20 years earlier.
    Do all these caveats mean that the hard work of Lorraine Pace and other community activists is for nothing? Not necessarily. Together with many other reports of breast cancer clusters on Long Island, the West Islip situation highlighted by Pace has helped epidemiologists lay the groundwork for a well designed scientific study.  
The passage suggests that the fact that "only one community cancer cluster has ever been traced to an environmental cause" (in the third paragraph) is most likely due to the ______.
选项 A、methodological difficulties in analyzing community cancer clusters
B、reluctance of epidemiologists to investigate environmental factors in cancer
C、lack of credibility of citizen activists in claiming to have identified cancer agents
D、effectiveness of regulations restricting the use of carcinogens in residential areas
答案A
解析 第三段指出，虽然几种致癌因素是通过这样的(指第二段提到的情况)职业或病理群体发现的，只有一种患癌群体被归因于环境所致。原因是，对于社会上怀疑环境因素(导致癌症)的说法，卫生部门的官员经常不予理睬，因为普通人经常连那些搬入某个社区之前就已经被诊断为患了癌症的人也包括在内。另外，癌症的潜伏期(latency)也增加了分析引起癌症的原因上的困难。可见，这里强调了分析癌症发病原因的困难，指出有些人的分析方法不对，忽视了一些因素。
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