What were you doing ______there was a knock at the door?A.while B.until C.when D.after
问题描述：
What were you doing ______there was a knock at the door?A.while B.until C.when D.after
答
C
指的是一个时点能具体讲解一下吗指在有人敲门的那一刻（过去），你在干什么。 这个从句所指的是个瞬间动作（用when）， 为过去进行时What were you doing 这个相对稳定的动作作时间铺垫。