Zachary has been very depressed for several weeks. He just started therapy and went to his therapist for the first time yesterday. Today, he says that while he is still a "little down," he feels much less depressed than he was. Which of the following pairs of factors are MOST likely to have contributed to his improved mood?

a. Placebo effect and regression toward the mean
b. Regression toward the mean and spontaneous recovery
c. Placebo effect and therapy
d. Regression toward the mean and therapy

A