① NaHCO₃＋NaOH=Na₂CO₃＋H₂O；

② NaOH＋CO₂=NaHCO₃；

③ 2NaOH＋CO₂= Na₂CO₃+H₂O；

④ CO₂＋Na₂CO₃＋H₂O =2NaHCO₃ 。

如图所示的“三角形”的三边分别表示了上述反应中的一个。

则：OA表示反应{#blank#}1{#/blank#}(选填①②③④等，下同)；

OB表示反应{#blank#}2{#/blank#}；BA表示反应{#blank#}3{#/blank#}。

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