Dr. Allen is studying the effects of stress on sexual desire. Dr. Allen's experimental design is to ...

Dr. Allen is studying the effects of stress on sexual desire. Dr. Allen's experimental design is to engage subjects in one of three experimental scenarios in which they play a game that is of easy, medium or high difficulty.
 
  The goal is to evoke low, medium, or high stress responses in subjects depending on which game they play. Following the game, the subjects are then asked to rate their level of desire for having intercourse with an attractive man or woman as depicted on a TV screen. Which part of Dr. Allen's experiment represents the dependent variable?
 
  a. the subjects ratings of their level of sexual desire
  b. the subjects themselves
  c. the level of stress evoked by the games
  d. Dr. Allen himself