6.98 Vitamin C and colds.

In a study of the suggestion that taking vitamin C will prevent colds, 400 subjects are assigned at random to one of two groups. The experimental group takes a vitamin C tablet daily, while the control group takes a placebo. At the end of the experiment, the researchers calculate the difference between the percents of subjects in the two groups who were free of colds. This difference is statistically signifcant in favor of the vitamin C group. Can we conclude that vitamin C has a strong effect in preventing colds? Explain your answer.