The Dihydrogen Phosphate Ion, H₂PO₄ ^- , Can Behave as Both

The dihydrogen phosphate ion, H₂PO₄ ^- , can behave as both an acid and as a base. What is the chemical formula of the conjugate acid of dihydrogen phosphate? What is the chemical formula of the conjugate base of dihydrogen phosphate?

A) conjugate acid = H₃PO₄ conjugate base = HPO₄^{2 -}
B) conjugate acid = H₂PO₄⁺ conjugate base = H₂PO₄^{2 -}
C) conjugate acid = HPO₄^{2 -} conjugate base = H₃PO₄
D) conjugate acid = H₃PO₄ conjugate base = PO₄^{3 -}
E) none of these