Bunuel wrote:
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.
Fact: The number of United States citizens suffering from the flu increased significantly this year over last year
Evidence: this increase did not occur among citizens of other countries
Conclution: it is likely the result of a particular flu virus confined to the United States
This is a weak argument. Author want to drive testtakers to the way that the number of people suffering from flu not increase, so cannot be the reason of other increase. But from "Not increase" to cannot be reason" needs a big assumption.
AnalogyAverage GMAT score of class A is increased from this to last year.
This increase does not happen with other class.
Conclution: this increase is the result of students in class A
There can be case that average of other class is 800, cannot be higher. And students in class A learn alot from other class.
Back to question, C break the argument. People bring flu from abroad to country.