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 subjects who did not exercise regularly during the study tended to have diets that were more unhealthy.', 'A person who exercises regularly is probably doing so out of concern for his or her own health.', 'A large number of the deaths recorded were attributable to preexisting conditions or illnesses.', 'Whether or not a given subject was to exercise during the study was determined by the researchers on a random basis.']