I rang you at about ten but there was no replyOh that was probably I was seei

问题补充：

--- I rang you at about ten, but there was no reply.

--- Oh, that was probably I was seeing the doctor.

A. for

B. why

C. when

D. how

答案：

A. for

B. why

(答案→)C. when

D. how

解析：考查连词。根据上下文可的句意知C正确。句意—我在10点打你电话，但是没有回音。—那也许就是在我去看医生的时候吧。