Hey Carmen — hope you don’t mind but I moved your Q to a new thread so that if others have the same question they can more easily find this in the future —
Per answer (A), which says that If N works 1, H works 2 —
We know that it two things must be true: If N is in 1, H is in 2 and the contrapositive: If H is in 1, N is in 2.
Per these rules, there are 3 possibilities for how N and H could be arranged: (N1, H2) (H1, N2) (N2, H2).
It is the third of these possibilities that makes it so that an “if…then” statement has a different consequence than a straight-up “P and H will work different shifts,” and that’s why (A) doesn’t have the same meaning that (C) does — (C), which is a biconditional (discussed on pg. 191), does not allow for them to work either shift together — (C) only allows for two options: (N1, H2) and (H1, N2) just like the original rule only allowed for two possibilities —
I hope that clears it up — good luck studying this week — Mike