Researchers Note That Regular Exercise Appears to Lessen the Detrimental

Multiple Choice

Researchers note that regular exercise appears to lessen the detrimental effects of stress; however, an alternative explanation could be that 

A) people who receive the strongest effects are already at peak conditioning.
B) people who already have less stress are more likely to go to the gym and exercise anyway.
C) people who are already physically healthy have maladaptive coping strategies.
D) all of these explanations are true.