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I don't understand how D. must be true. I thought o could be hired the same year as O in '90 and that would make it come before R in '91 because two professors could be hired in the same year and since we don't know the specialty, that should work, shouldn't it?

The problem # would help..

Saying the answer D only appears in game 4 once, I'm assuming you mean question #21. N shares a specialty with R, which if you combine that with P and S come in between N and M, you're forced to place N in 89. Since S must come between 89 and 93, that leaves 90, 91, and 92 to place him. But, the last rule says that S shares a specialty with O. If O is in 90, then S cannot be in 90 or 91, therefore the only possible slot to place S is 92. Since S must be in 92 and R is in 91, R will always be hired before S.