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Dear friends
Can you provide the answer?

Here's the question.

Do you know________?
1. What happened
2. who lives there
3. who at the door is
4. when didit happen

I know the answer should be no. 1
But I don't know why?

Thanks for your helping!

Charis
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Is there a typo in 4?

Assuming that 4 is intended to read "when did it happen"...

1 would have been perfectly acceptable had it had a lowercase 'w'
2 is fine as it is
3 would only be acceptable in poetry, not in normal English
4 would only be acceptable in poetry, not in normal English

Rommie