People who eat losts of fruits and vegetables have lower rates of colon cancer than those who eat little of these foods.

Question

Fruits and vegetables are rich in antioxidants such as vitamins A, C, and E.
Will taking antioxidants help prevent colon cancer?
A medical experiemtn divided subjects into four gruops:daily beta-carotene, daily vitamins C and E, all three vitamins every day or daily placebo.

1. What are the explanatory and response variables in this experiment?

My question: Is the placebo considered an explanatory variable??

I know the beta-carotene, vitaminds C and E, and all three vitamins are. The response is the incidence of colon cancer.