He had just sent an e­mail to his pal on the Internet the system broke down.A. whileB

问题补充：

He had just sent an e­mail to his pal on the Internet the system broke down.

A. while

B. when

C. after

D. since

答案：

A. while

(答案→)B. when

C. after

D. since

解析：句意：他刚刚在网上发给他的网友一封电子邮件，电脑的系统就出故障了。while在……期间，表示时间段； when在这时，表示时间点； after在……之后；since 自从。根据句意表示正在此时，因此用连词when。故选B。