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题目材料：
A recent study shows that farmers have a 40 percent lower risk of developing various common forms of cancer than people in the general population do. Farmers are more active than most people are, but even taking this into account, farmers had a significantly lower cancer risk. Therefore, it is likely that some substances that farmers are typically exposed to in the course of their work lower the risk of developing these forms of cancer.
Which of the following, if discovered to be true, would cast the most serious doubt on the argument?
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A.There are types of cancers against which physical exercise has no known protective effect.
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B.In the group that remains after urban dwellers are eliminated from the study' s sample, the risk of developing cancer is the same for farmers and nonfarmers.
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C.There are types of cancers that occurred with equal frequency among the farmers and nonfarmers in the study' s sample.
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D.Most of the substances that farmwork exposes a farmer to are typically present in the work environment of one or another nonfarming occupation.
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E.Many farmers in the study' s sample were exposed to pesticides that have been shown to increase the risk of cancer.
B显示答案
