题目内容
（请给出正确答案）
[主观题]
The passage suggests that the fact that "only one community cancer cluster has ever been t
raced to an environmental cause" (in the third paragraph) is most likely due to the ______.
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
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