Consider the following research summary:

"To test the widespread idea that chocolate is harmful in instances of acne vulgaris, five normal subjects ingested two enriched chocolate bars daily for one month; this represented a daily addition of the diet of 1,200 calories, of which about half was vegetable fat. Measurements of the subject's output of facial sebum before and after this period indicated no effect caused by the additional chocolate intake."

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.