A researcher finds out 2000 mildly overweight women who exercise regularly, have not had heart attack, and are willing to participate in the study. She randomly assigns 500 of women to take appetite suppressant. The other 500 women are given the placebo. Both groups are followed for 5 years and amount of weight lost after this time is recorded.
The factor in experiment is
A. The weight lost by women in the study.
B. Use of a control group taking a placebo.
C. The treatment taken by women in the study - appetite suppressant or placebo.
D. The length of the study.