Three reactions involving ${\mathrm{H}}_2{\mathrm{PO}}_4^{-}$ are given below

$\left(\mathrm{I}\right) {\mathrm{H}}_3{\mathrm{PO}}_4^{ } + {\mathrm{H}}_2\mathrm{O} \to {\mathrm{H}}_3{\mathrm{O}}^{+} + {\mathrm{H}}_2{\mathrm{PO}}_4^{-}$

$\left(\mathrm{II}\right) {\mathrm{H}}_2{\mathrm{PO}}_4^{-} + {\mathrm{H}}_2\mathrm{O} \to {\mathrm{HPO}}_4^{2-} + {\mathrm{H}}_3{\mathrm{O}}^{+}$

$\left(\mathrm{III}\right) {\mathrm{H}}_2{\mathrm{PO}}_4^{-} + {\mathrm{OH}}^{-} \to {\mathrm{H}}_3{\mathrm{PO}}_4^{ } + {\mathrm{O}}^{2+}$

In which of the above does ${\mathrm{H}}_2{\mathrm{PO}}_4^{-}$ act as an acid ?

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