The number of United States citizens suffering from the flu increased significantly this year over last year. Since this increase did not occur among citizens of other countries, it is likely the result of a particular flu virus confined to the United States.
Which of the following, if true, would cast the most serious doubt on the explanation for the increase given above?
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AMany non-US citizens took flu vaccines this year.
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BThis year, the flu suffered by citizens of other countries was generally more severe than that suffered by US citizens.
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CA large percentage of US citizens who suffered from the flu this year were living abroad.
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DMany US citizens who suffered from the flu this year also suffered from the flu last year.
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EThe number of US citizens did not change significantly between last year and this year.
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显示答案
正确答案: C