【单选题】The passage suggests that the fact the 'only one community cancer cluster had ever been traced to an environmental cause' (Para. 3)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