Explanation of this dialogue

[Emma]

English Grammar Tests, Elementary Level

ESL/EFL Test #33 "Responses (6)", question 4

John: 'That was very thoughtful of you to do that.'
Sue: '.........'

(a) Forgetful.
(b) Forget it.
(c) I've forgotten.
(d) Don't forget it.

English Grammar Tests, Elementary Level

ESL/EFL Test #33 "Responses (6)", answer 4

John: 'That was very thoughtful of you to do that.'
Sue: 'Forget it.'

Correct answer: (b) Forget it.

Your answer was: ?
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i simply can't understand the question what does it mean?

Emma

[Alan]

Hi,

The explanation of this dialogue:

[Snoopy_Blah_Blah]

Is forget it a really polite way to answer? Could it have had a hint of dissapointment, because I either didn't understand what a pesron was trying to say or pretended to not understand, for it was rather unpleasant. And in that case forget it sounded bitter.
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Everything happens for a reason

[Conchita]