A study showed that people who take vitamin C daily have a significantly better chance of protecting against a cold than those who did not take the vitamin daily. Which of the following statements is correct?

I. This is an observational study and not an experiment.

II. This study provides evidence that taking vitamin C daily contributes to protecting against a cold.

III. The general health consciousness of people who take vitamin C could be a confounding variable.

a. I only

b. III only

c. I and III only

d. II and III only

e. I, II, and III