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We .................................... rushed to the airport as the plane was delayed for several hours.

a) mustn't have b) may not have c) needn't have d) shouldn't have

I couldn't choose anyone but i know it is a or c. Although it is very question, i'm confused from time to time.
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Actually, I think it's a toss-up (meaning it could go either way) between c and d, with c slightly ahead.

Would it help you decide if it were re-phrased?

We were afraid we'd be late meeting the plane, so we rushed to the airport. But when we go there, we found out the plan was delayed for several hours. So even though we rushed, we ________.
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Yes, c is the most reasonable choice, because is the weakest verb, IMO, and that's what we need here.