was it 8 o'clock _____you heard someone _____ at the door?a.when;knockingb.when;knockc.that;knockd.that;knocking请略解

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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

A

a

C.

A