An experimental treatment was developed to improve memory retention in adults. To test its effectiveness, a sample of 120 participants was selected, and each participant's memory was measured before receiving the treatment. Two months after completing the treatment, each participant's memory was measured again, and comparisons were made between the before and after-treatment measurements. What type of experimental design is being used?
a. Stratified
b. Matched pair
c. Randomized block
d. Completely randomized