D
Psychologists have claimed that many people are more susceptible to psychological problems in the winter than in the summer; the psychologists call this condition seasonal affective disorder. Their claim is based on the results of surveys in which people were asked to recall how they felt at various times in the past. However, it is not clear that people are able to report accurately on their past psychological states. Ttherefore, these survey results do not justify the psychologists' claim that there is any such condition as seasonal affective disorder. The author criticizes the psychologists' claim by
A: offering an alternative explanation of the variation in the occurrence of psychological problems across seasons
B: demonstrating that fewer people actually suffer from seasonal affective disorder than psychologists had previously thought
C: questioning the representativeness of the population sample surveyed by the psychologists
D: questioning an assumption that the author attributes to the psychologists