節錄Collins Cobuild
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Not at all is a polite way of acknowledging a person's thanks.

ex:
'Thank you very much for speaking with us.'
*`Not at all.'

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Not at all is an emphatic way of saying `No' or of agreeing that the answer to a question is `No'.
ex:
`Sorry. I sound like Abby, don't I?'*`No. Not at all.'...

`You don't think that you've betrayed your country.'*`No I don't. No, not at all.'