A recently completed study of several hundred subjects, all of approximately the same age, showed that those who exercised regularly during the study were much less likely to die during the study. This indicates that exercise can actually increase one’s life span.
Which one of the following, if true, most strengthens the argument?The argument concludes that exercise increases life span because the people who exercised were less likely to die during the study.
The main weakness is that exercisers may have been healthier for some other reason. The best strengthener should show that exercise itself, not some preexisting difference, caused the survival difference.
(A) The subjects who did not exercise regularly during the study tended to have diets that were more unhealthy.
Wrong. This weakens the argument because poor diet, not lack of exercise, could explain the higher death rate.
(B) The subjects who did not exercise regularly during the study tended to blame their lack of exercise on a lack of time.
Wrong. The reason they did not exercise does not show that exercise caused the survival difference.
(C) A large number of the deaths recorded were attributable to preexisting conditions or illnesses.
Wrong. This weakens the causal claim because the deaths may have been caused by prior health problems rather than lack of exercise.
(D) Whether or not a given subject was to exercise during the study was determined by the researchers on a random basis.
Correct. Random assignment makes it less likely that the exercising group was healthier to begin with. So if the exercising group survived at a higher rate, that supports the idea that exercise itself
increased life span.
(E) A person who exercises regularly is probably doing so out of concern for his or her own health.
Wrong. This weakens or at least does not help, because people concerned about health may also have other healthy habits.
Answer: (D)