verb
BAD : It's time the government reacted on the crisis.
GOOD : It's time the government reacted to the crisis.
BAD : Their reaction on the article was predictable.
GOOD : Their reaction to the article was predictable.
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react/reaction to sth (NOT on/at ): 'People reacted to the speech in different ways.' 'Reactions to the news of the merger were less positive than expected.'