How can biologists find out whether Vitamin C prevents colds

How can biologists find out whether Vitamin C prevents colds? Vitamin C has been a common remedy for colds for many years. However, the study has shown that Vitamin C has no effect on cold. Biologists have carried out controlled experiments which be followed the steps below to investigate it.

Firstly, biologists recruit many volunteers and put them into two groups. Then, people in one of the group take 2 tablets of Vitamin C daily for one year, otherwise the other group which called controlled group only have placebos. Both of them must record truthfully the times of getting cold in the year. In the end, biologists should collect data and compare the results of two groups. If the study shows that people who took up Vitamin C daily caught colds at the same rate as people who took a placebo, we can get the conclusion that Vitamin C does not work to cold. If not, we say that Vitamin C may prevent colds. However, we cannot be sure of it, it need further experiments.