--When did you hear the gunshot last night?---________ was at about 10 o’clock ________ I

--When did you hear the gunshot last night?---________ was at about 10 o’clock ________ I
--When did you hear thegunshot last night?
---________was at about 10 o’clock ________ I was just about to go to bed.
A.It; when B.It; that C.That; when D.That; that
这题答案是A ,
我的疑问：1.为啥不能选择B ,是强调句； 2.为啥不能用that?