For a certain experiment, the available experimental units are six people: three females (f) and three males (m). Use (1f, 2f, 3f) to denote the females and (1m, 2m, 3m) to denote the males. There are to be three treatment groups: A, B, and C. If a randomized block design is used, with the experimental units blocked by gender, which assignment of treatments is not correct? (5 points) Select one:
a. A: (1f, 1m), B: (2f, 2m), C: (3f, 3m)
b. A: (2f, 1m), B: (3f, 2m), C: (1f, 3m)
c. A: (1f, 2m), B: (3f, 2f), C: (3m, 1m)
d. A: (2f, 3m), B: (3f, 2m), C: (1f, 1m)