Researchers are investigating whether taking aspirin regularly reduces the risk of heart attacks. Four hundred men participate in the study. The men are divided randomly into two groups: one group takes aspirin pills, and the other group takes placebo pills (a pill with no aspirin in it). The men each take one pill a day, and they do not know which group they are in. At the end of the study, researchers will count the number of men in each group who have had heart attacks. Identify the explanatory and response variables in this situation.
A. Explanatory variable: whether a subject had a heart attack, Response variable: the type of pill the men took each day
B. Explanatory variable: the type of pill the men took each day, Response variable: whether a subject had a heart attack
C. Explanatory variable: the 400 men participating in the study, Response variable: whether a subject had a heart attack
D. Explanatory variable: the aspirin pills, Response variable: the placebo pills (containing no aspirin)