问题补充：
----Have you seen Mr. Black these days, Jack?
----Yes, I met him when I in the street yesterday.
A. walked
B. was walking
C. have walked
D. is walking
答案：
A. walked
(答案→)B. was walking
C. have walked
D. is walking
解析：在when 引导的时间状语从句中，when引导的从句的动词可以为延续性动词，也可以是短暂性动词，表示主句的动作发生在从句的动作之中，故选B
