the boy couldnt enter his house.A. Since the key had lostB. The key being lostC. Lost

问题补充：

, the boy couldn enter his house.

A. Since the key had lost

B. The key being lost

C. Lost the key

D. Having lost the key

答案：

A. Since the key had lost

(答案→)B. The key being lost

C. Lost the key

D. Having lost the key

解析：主语不同，分词构成独立主格。不选D, 因为句中强调的不是先后顺序，而是强调原因。