Consider the following research summary:

"To test the widespread idea that chocolate is harmful in instances of acne vulgaris, 65 subjects with moderate acne ate either a bar containing ten times the amount of chocolate in a typical bar, or an identical-appearing bar which contained no chocolate. Counting of all the lesions on one side of the face before and after each ingestion period indicated no difference between the bars."

a) What is the treatment? What are the treatment and control groups?

b) What are the explanatory and response variables?

c) Discuss one possible limitation of this study.