He left the key he had been an hour before.A. whatB. thatC. whereD. which

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A. what
B. that
(答案→)C. where
D. which
解析：考查状语从句。句意：他把钥匙忘记在了一小时前他所在的地方。本句中的where he had been an hour before在句中做地点状语，修饰动词left the key。故C正确。