When we --an English movie,the headmaster--past our classroo

When we __an English movie,the headmaster__past our classroom. 是选择题 选项在下面
A watch;walked
Bwere watching;was walking
Cwatched;was walking
Dwere watching;walked
麻烦各位高手帮帮忙啊
感激不尽啊
顺便解释一下
谢谢
答案是选D
麻烦解释一下为什么要选D
谢谢