I first met your sister ten years ago, when she at a supermarket as a salesgirl.
A. has worked
B. had worked
C. was working
D. had been working
网友回答
A. has worked
B. had worked
(答案→)C. was working
D. had been working
解析：过去进行时的用法：过去某个时间点或者时间段正在发生的事情。“十年前我初次见到你姐姐的时候她在一家超市当服务员。”选C。