Question

A researcher tests the hypothesis that large, daily doses of vitamin c help protect against catching the common cold. what would be the best experimental and control group to test this hypothesis?

Answer 1

The best thing to do is this: 
the subjects in the experimental group should be given large, daily dose of vitamin C while the control group subjects should be given a daily dose of sugar pills that are disguised as vitamin C.
The research can then observe the differences between the two groups.