In a Study of Aggression, Dr

In a study of aggression, Dr. Berkowitz had participants watch either violent or non-violent media after they had been frustrated by a colleague. They were then able to deliver electric shocks to the colleague as feedback for mistakes made on a memory test.
Berkowitz recorded the intensity of shocks delivered. Results indicated that more intense shocks were given when participants watched the violent media. In this study, the shocks delivered are

A) a measurement error
B) the confounding variable
C) an interaction effect
D) the independent variable
E) the operational definition of aggression