-could you tell me () at that time?-I was Watching TV at hom

-could you tell me () at that time?-I was Watching TV at home.
-could you tell me () at that time?
-I was Watching TV at home.
A.what you were doing
B.what were you doing
C.what are you doing
D.what you are doing
为什么不是B啊..【could在这里不是过去时了麽,为什么问句中可以出现两个过去时的单词?】