The agent was about to sign his name to the document to our surprise a stranger broke in

问题补充：

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

答案：

A. while

B. when

C. as

D. until

解析：B

解析when此处为并列连词，相当于at that time，可译为“这时(突然)……”。