备用钥匙
先理解核心含义，再看它在真实句子里的用法。
备用钥匙
a duplicate key kept in case the original is lost or locked inside
I keep a spare key under the doormat.
我把备用钥匙放在门垫下面。
备用钥匙
a duplicate key kept in case the original is lost or locked inside
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Dictionary facts from AI_SUPPLEMENT.