Men and women respond differently to a certain medication. An experiment to compare the effectiveness of three levels of this medication looked separately at the reactions of men and women. A sample of 200 people were selected and then divided by gender. Within each group, subjects were randomly assigned to receive one of the three medication levels. What type of experimental design is being used?
a. Matched pair.
b. Completely randomized.
c. Stratified.
d. Randomized block.