What is the formula of the conjugate base of the hydrogenphosphate ion, \({\rm{HPO}}_4^{2 - }\)?

A.     \({{\text{H}}_{\text{2}}}{\text{PO}}_4^ - \)

B.     \({{\text{H}}_{\text{3}}}{\text{P}}{{\text{O}}_{\text{4}}}\)

C.     \({\text{HPO}}_4^ - \)

D.     \({\text{PO}}_4^{3 - }\)

D