The National Institutes of Health (NIH) set up a study to determine whether large doses of vitamin C

The National Institutes of Health (NIH) set up a study to determine whether large doses of vitamin C would shorten the length of time it takes to recover from a cold. Three thousand volunteers were split into two groups. For two weeks, members of Group A took 3,000 mg of vitamin C daily. Group B received 3,000 mg of a placebo. At the end of the two-week period, the researchers inserted live cold viruses directly into the noses of all the volunteers. The volunteers in both Group A and B continued to take their daily pills. All the volunteers got colds, and there was no significant difference in the length of time the colds lasted.


To have confidence that the results of the experiment were valid, you'd also want to know
   a whether the volunteers all worked for the same company.
   b. the genders of the volunteers.
   c. whether the volunteers exercised daily.
   d. whether any volunteers had colds at the start of the experiment.
   e. what the volunteers ate during the experiment.