Thirty overweight males agree to participate in a study of the effectiveness of three weight-loss treatments: X, Y, and Z. The researcher calculate the body fat composition of each subject. The response variable is the reduction in body fat after six weeks of treatment. Previous studies have shown the effects of a diet may vary based on a subject's initial body fat composition. How should the subjects be blocked? Explain.

Answer choices: They should be blocked alphabetically so the subjects are as different as possible.

They should be blocked by age because weight loss is more difficult to achieve as you get older.

They should be blocked by height because weight loss is easier to achieve for taller people.

They should be blocked by body fat composition so the subjects are as similar as possible.

Blocking should not be used in this experiment.