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Caustically
Negative Tone
Using 'caustically' expresses a sharp, cutting tone, suggesting strong disapproval or disdain. 
Harsh Sarcasm
'Caustically' often implies that the sarcasm is hurtful or intended to sting. 
Confrontational Context
Use 'caustically' for situations where someone is being confronted or challenged, not light banter. 