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。