For a certain experiment, the available experimental units are eight people: four females (f) and four males (m). Use (1f, 2f, 3f, 4f) to denote the females and (1m, 2m, 3m, 4m) to denote the males. There are to be four treatment groups: A, B, C, and D. If a randomized block design is used, with the experimental units blocked by gender, which assignment of treatments is correct? A: (1f, 3m), B: (2f, 2m), C: (3f, 1m), D: (4f, 4m)
A: (2m, 1m), B: (1f, 2f), C: (3f, 3m), D: (4f, 4m)
A: (3m, 1m), B: (2f, 4f), C: (3f, 1f), D: (4m, 2m)
A: (1f, 3f), B: (2f, 2m), C: (4f, 1m), D: (4m, 3m)