One of the Problematic Aspects in the Featured Study by Pennix

One of the problematic aspects in the Featured Study by Pennix and his colleagues was

A) the use of four separate control groups within the main part of the study
B) the age of the group studied
C) the fact that the subjects were followed for two years to determine the long-term impact of their exposure to the stressor
D) the fact that only 10% of the subjects developed colds after exposure to a respiratory virus