# Three reaction involving $H_2PO_4$ are given below $(i)H_3PO_4+H_2O \rightleftharpoons H_3O^{+}+H_2PO_4^{-} \\ (ii)H_2PO_4+H_2O \rightleftharpoons HPO_{4}^{-2}+H_3^{+}O \\ (iii)H_2PO_4+OH \rightleftharpoons H_3PO_4+O^{2-}$ In which of the above does $H_2PO_4$ act as an acid
$\begin{array}{1 1} (a)\;(i)\;only \\(b)\;(ii)\;only\\ (c)\;(iii)\;only \\ (d)\;(iv)\;only \end{array}$