Does Taking Large Amounts of Vitamins Protect Against Cancer? to Study

Does taking large amounts of vitamins protect against cancer? To study this question, researchers enrolled 29,000 Finnish men, all smokers over the age of 50. Half of the men, selected at random, took vitamin supplements, and others took a dummy pill that has no active ingredient. The researchers followed all the men for eight years. At the end of the study, men in the vitamin group were no less likely to have cancer than men in the other group. This study cast doubt on the popular idea that taking lots of vitamins can reduce the risk of cancer.
The study design looked like this:
The response variable should be named in the outline at "Question C." The response variable in this study is

A) whether a subject took vitamins.
B) 29,000 Finnish men.
C) random allocation.
D) a confidence statement.
E) whether a subject developed cancer.