Megumi is running an experiment on the impact of noise level on subsequent aggressive behavior. In one condition, participants experience a loud, annoying noise while trying to complete a task, and then are given a chance later to give another ostensible participant a noise blast, too. In the other condition, the participants do not experience the noise while trying to complete their task. After the study has been conducted, Megumi realizes that the loud room was lit more brightly than the quiet room. This difference in the level of light is an example of a a. confound.

b. control group.

c. threat to external validity.

d. subject variable.