The famous scientist was going out he found himself surrounded by lots of young people.A

问题补充：

The famous scientist was going out he found himself surrounded by lots of young people.

A. when

B. before

C. while

D. after

答案：

(答案→)A. when

B. before

C. while

D. after

解析：考查固定搭配，was/were doing….when….，刚要做什么，这时，句意：著名的科学家刚要出去，这时他发现自己被一群年轻人围住了，选A。