Hydrogen phosphate ions are dissolved in distilled water. Which of the following reactions correctly demonstrates the second equivalence point when this solution is titrated with hydrochloric acid?
a) H3PO4 + HCl <----> H2PO4 + H2O
b) HPO4 + HCl <----> H2PO4 + H2O
c) HPO4 + H3O <----> H2PO4 + H2O
d) H2PO4 + H3O <----> H3PO4 + H2O