4. The dihydrogen phosphate ion, \( \mathrm{H}_{2} \mathrm{PO}_{4}{ }^{-} \), is amphiprotic. In which of the following reactions is this ion serving only as a base?
(i) \( \mathrm{H}_{3} \mathrm{O}^{+}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{PO}_{4}^{-}(\mathrm{aq}) \rightarrow \mathrm{H}_{3} \mathrm{PO}_{4}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\ell) \)
(ii) \( \mathrm{H}_{3} \mathrm{O}^{+}(\mathrm{aq})+\mathrm{HPO}_{4}{ }^{2-}(\mathrm{aq}) \rightarrow \mathrm{H}_{2} \mathrm{PO}_{4}^{-} \)(aq) \( +\mathrm{H}_{2} \mathrm{O}(\ell) \)
(iii) \( \mathrm{H}_{3} \mathrm{PO}_{4}(\mathrm{aq})+\mathrm{HPO}_{4}{ }^{2-} \) (aq) \( \rightarrow 2 \mathrm{H}_{2} \mathrm{PO}_{4}{ }^{-} \)(aq)
(a) i only
(b) i and ii
(c) i and iii
(d) ii and iii
(e) i, ii, and iii