Grammatically, these are all correct. I would have to guess they are looking for "A" because the other phrases all include "were supposed to be," which implies advance planning. However, I can easily imagine a scenario for D: the director, John, angrily leaves the theater in the middle of a rehearsal.
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HeiditaOnly a is correct.I agree with you. Only (a) is correct.
cheers[H]
Anonymous Here we talk about an accidental situationThere is no evidence that it was accidental