The key ___,he could not enter the room.A having been lost B

The key ___,he could not enter the room.A having been lost B had been lost C having lost D losing

首先确定,钥匙肯定是被掉的,就是肯定用be lost这个结构,
CD都是主动语态,错误
再分析,因为逗号后面没有连词,说明前面不是完整的句子,只能是分词作伴随,表示原因,
既然是分词,那么填的答案肯定是doing开头
所以,答案A
现在分词的被动语态
be lost
be 用 doig 代替,就是being,
再看时态,应该是先掉钥匙,然后进不了房间,
所以,be 用 have done 代替
就是,have been lost
又因为要分词
所以,having been lost