In a repeated-measures study, the same group of individuals participates in all of the treatment conditions. Which of the following situations is not an example of a repeated-measures design?

a. A researcher would like to study the effect of practice on performance.

b. A researcher would like to compare individuals from two different populations.

c. The effect of a treatment is studied in a small group of individuals with a rare disease by measuring their symptoms before and after treatment.

d. A developmental psychologist examines how behavior unfolds by observing the same group of children at different ages.