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I read this question somewhere before and have been puzzled about the original answer of 'couldn't':
---Have some more cake.
---Oh, I couldn't/ can't. Thank you.
I thought both answers are all right and I'm more likely to say 'I can't'.
Do both 'couldn't' and 'can't' sound all right in the reply? Could you explain why?
I was hoping a native English teacher could give me the answer. Thank you very much in advance.
why not 'can't' but 'couldn't'? 
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