was it 8 o'clock _____you heard someone _____ at the door?

问题描述:

was it 8 o'clock _____you heard someone _____ at the door?
a.when;knocking
b.when;knock
c.that;knock
d.that;knocking
请略解

A,不选C的理由是这并不是一个it is...that...句型,如果在8'clock前面加个at那就选C了