I didn't know when they （ ） for beijingA.leftB.leaveC.are leavingD.were leaving

I didn't know when they （ ） for beijingA.leftB.leaveC.are leavingD.were leaving
I didn't know when they （ ） for beijing
A.left
B.leave
C.are leaving
D.were leaving

I didn't know when they （ ） for beijingA.leftB.leaveC.are leavingD.were leaving
A.
我不知道他们什么时候动身去的北京.
过去时了.主句都是过去时.

D

A.
didn't后面跟过去式，排除掉BC
leave是瞬间动词，不能用were leaving