Fitz is testing the effectiveness of a smoking cessation intervention in a sample of heavy smokers. The intervention involves using classical conditioning techniques to make the participants find smoking unpleasant (the control group does not receive the intervention). The results show that the intervention was effective; the participants in the treatment group had higher quitting rates than those in the control group. However, Fitz realized that the participants in the control group had started smoking at a younger age than those in the treatment group, making quitting more difficult. Therefore, age of smoking initiation was a(n):

a. confound
b. moderator
c. third variable
d. mediator

Answer: a. confound