Teacher: Is chocolate sour?
Pupil A: No, it isn't. It is sweet.
Pupil B: No, it is sweet.
Is Pupil B correct to leave out "it isn't" ?
Both replies are fine.
Neither is any less polite than the other.
not a teacher
Unless the teacher stipulates that a sentence is required, a more common answer would be 'No'.
Rover
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