题目选项分析

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?

【选项】A large percentage of US citizens who suffered from the flu this year were living abroad.