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