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気にする is transitive (する is transitive here), so you use を: 私は(が*)それを気にしません。 The を can be replaced by は when それ is thematic or contrasted: それは私は(が*)気にしません。 As an aside: Grammatically speaking you can use それが with intransitive 気になる (なる is intransitive), as in: (私は/私には)それが気になりません。 *For the difference of が and は, this thread might be of help: What&#...


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