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

Multiple Choice

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:
An example of a confounding variable that might affect the results of this study is

A) the brand of cigarette smoked.
B) the age of participant.
C) the brand of vitamin supplement.
D) none of the above because the subjects were divided up randomly, and any possible effects due to confounding variables should be balanced between the two groups.