For a certain experiment, the available experimental units are eight rats, of which four are female

(F1, F2, F3, F4) and four are 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 S (F1, M1), B S (F2, M2), C S (F3, M3), D S (F4, M4)

(b) A S (F1, M2), B S (F2, M3), C S (F3, M4), D S (F4, M1)

(c) A S (F1, M2), B S (F3, F2), C S (F4, M1), D S (M3, M4)

(d) A S (F4, M1), B S (F2, M3), C S (F3, M2), D S (F1, M4)

(e) A S (F4, M1), B S (F1, M4), C S (F3, M2), D S (F2, M3)