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 groups 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.