Which of the following research studies would provide the best evidence for a cause-and-effect relationship between using a specific supplement and reduced risk for the common cold?
A. A cross-sectional study of 200 people that found that current users of the supplement were 50% less likely to report having an upper respiratory infection at the time of the study.
B. A laboratory study on human cells that found that the supplement affected the activity of specific types of immune cells.
C. A randomized clinical trial following 200 people for 6 months found that those receiving the supplement had 50% fewer upper respiratory infections compared to those receiving a placebo.