For a certain experiment, the available experimental units are eight rats, of which four are female (F1, F2, F3, F4) and four ar
e male (M1, M2, M3, M4). 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 of the following assignments of treatments is impossible?
a. A (F1, M1), B (F2, M2), C (F3, M3), D (F4, M4)
b. A (F1, M2), B (F2, M3), C (F3, M4), D (F4, M1)
c. A (F1, M1), B (F3, F2), C (F4, M1), D (M3, M4)
d. A (F4, M1), B (F2, M3), C (F3, M2), D (F1, M4)
e. A (F4, M1), B (F1, M4), C (F3, M2), D (F2, M3)