4. The concept of "individuals serve as their own control" with respect to a repeated- measures design means:

a. The same group of individuals is assigned to both treatment and control groups simultaneously.

b. Individuals receive all treatments, which eliminates individual differences such as age, IQ, and education among participants.

c. Individuals within a group are in control of what treatment they receive.

d. Individuals who receive a treatment are in control over how the treatment will be administered, measured, and compared.