The agent was about to sign his name to the document , to our surprise, a stranger broke in to prevent him.
A. while
B. when
C. as
D. until
网友回答
A. while
(答案→)B. when
C. as
D. until
解析：考查连词：be about to…可以和when…连用，表示“刚要做…这时…,”句意：代理刚要在文件上签名字，这时使我惊讶的是，一个陌生人闯进来阻止了他。A. while当…时候，表示一段时间，C. as 当…时候，表示两个动作同时发生，D. until直到，选B。