--When did you hear the gunshot last night?----------- was a

--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?