Does Taking Vitamins Prevent Colon Cancer? a Study Assigned 864

Multiple Choice

Does taking vitamins prevent colon cancer? A study assigned 864 subjects at random to four groups. One group took beta-carotene, another took vitamins C and E, a third took all three, and the fourth group took only a dummy pill. After four years, there was no difference among the groups in the formation of polyps in the colon that precede cancer.
Does the new study (properly designed) give good reason to think that vitamins prevent colon cancer?

A) Yes, randomized comparative experiments give good evidence for causation
B) No, there was no significant difference among the groups.
C) Yes, a study incorporating a placebo gives good evidence for causation.
D) No, this is an observational study.