Having lost his key， .
A. he couldn’t enter the room
B. the gatekeeper offered to help
C. the door was forced open
D. the room couldn’t be entered
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(答案→)A. he couldn’t enter the room
B. the gatekeeper offered to help
C. the door was forced open
D. the room couldn’t be entered
解析：考查句子结构：逗号前面是现在分词的完成式做状语，说明句子的主语应该是lost his keys的逻辑主语，应该是he，句意：丢了钥匙后，他不能进入房间。选A。