I first met Lisa three years ago when we------at a radio sta

I first met Lisa three years ago when we------at a radio station together .i first met Lisa three years ago when we------at a radio station together .A have worked B had been working C were working D had worked 我知道答案是C 但是我们老师讲的时候说when后面要加过去形式 while后面加过去进行时 如果这么说应该填worked 但是为什么是was working