核心问题
Just then there was a knock on the door.
我认为 a knock on the door 是错误的,应改为 a knock at the door。
词典解答
a knock 之后通常接 at, 但有时也接 on。例如：
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There was a knock at the door and in came Susan. 有敲门声,走进来的是苏珊。
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She gave two knocks on the door. (CLD) 她在门上敲了两下。