Three reactions involving H₂PO₄⁻ are given below:

(i) H₃PO₄ + H₂O H₃O⁺ + H₂PO₄l⁻
(ii) H₂PO₄⁻ + H₂OHPO₄²⁻ + H₃O⁺
(iii) H₂PO₄⁻ + OH⁻H₃PO₄ + O²⁻

In which of the above does H₂PO₄⁻ act as an acid?

(a) (i) only
(b) (ii) only
(c) (i) and (ii)
(d) (iii) only