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?
A. Many non-US citizens took flu vaccines this year.
B. This year, the flu suffered by citizens of other countries was generally more severe than that suffered by US citizens.
C. A large percentage of US citizens who suffered from the flu this year were living abroad.
D. Many US citizens who suffered from the flu this year also suffered from the flu last year.
E. The number of US citizens did not change significantly between last year and this year.
Which of the following, if true, would cast the most serious doubt on the explanation for the increase given above?
A. Many non-US citizens took flu vaccines this year.
B. This year, the flu suffered by citizens of other countries was generally more severe than that suffered by US citizens.
C. A large percentage of US citizens who suffered from the flu this year were living abroad.
D. Many US citizens who suffered from the flu this year also suffered from the flu last year.
E. The number of US citizens did not change significantly between last year and this year.
