Thinking Skills Assessment: Entrance Exam Sample Questions
In European countries the incidence of cancer is higher than in non-European developing countries. This difference in cancer rates cannot be due to a greater genetic susceptibility to cancer amongst Europeans because statistics show that Europeans who migrate to developing countries decrease their chances of getting cancer and that those from developing countries who migrate to Europe increase their chances of getting cancer. Since there are major differences in diet between Europe and developing countries it is reasonable to conclude that the higher cancer rates in Europe are due to certain cancer-causing foods.

Which one of the following if true would most weaken the above argument? 

Correct!
C
Stress is a contributory cause of cancer and Europeans experience higher than average stress in their daily lives.
Explanation
The argument is claiming there is a correlation and probable causal relationship between diet and food based upon a major difference in rates between different people living in different places. Though the food can be different between Europe and elsewhere, so are many other things - climate, stress etc..  As with reasoning from correlation in other cases - and it is always true that correlation is necessary, but not sufficient for causation, there can always be a third or hidden variable that can account for the correlation. Hence C is the best choice.

A is not relevant for the argument's conclusion about European rates.

B is not relevant for the conclusion - it is not related to diet's so called correlation with cancer rates.

D could be true; but, it is not relevant to the conclusion, which is only speaking about cancer rates.

E This could be true, but, the argument is claiming that ethnicity is not a factor (second line and line 5).