Joann conducted an experiment to test the effectiveness of an anti-anxiety program. The experiment took place over a 1-month time period.

Participants in the control group and the experimental group (those who participated in the antianxiety program) recorded their anxiety levels several times each day. Joann was unaware that midterm exams also happened to take place during the 1-month time period of her experiment.

Joann’s experiment is now confounded by:

a. a maturation effect.

b. a history effect.

c. regression to the mean.

d. a mortality effect.