B would be correct if it was "if we had left" It is counterfactual. "If we left" can be used as a hypothetical proposition before a future event. The play starts at 8:00.
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sb70012Why can't B work?I've heard people use that grammatical pattern, and they probably do it because we so often substitute the past for the past perfect. Nevertheless, it is not careful English, and careful English is normally expected in an exam setting.