"Vitamin C supplements prevent colds. A study found that people who take Vitamin C have fe

A) Vitamin C is inexpensive

B) People who take supplements tend to be more health-conscious overall, exercising more and eating better

C) Some people are allergic to Vitamin C

D) Vitamin C occurs naturally in citrus fruits

Explanation

Health-conscious behavior is a confounding variable. The fewer colds may be due to overall healthier lifestyles, not the Vitamin C specifically.