The conjugate base of ${\mathrm{H}}_2{\mathrm{PO}}_4^{-}$ is ;

• A

  ${\mathrm{H}}_3{\mathrm{PO}}_4$

• B

  ${\mathrm{H}}_2{\mathrm{PO}}_4^{-}$

• C

  ${\mathrm{HPO}}_4^{2-}$

• D

  ${\mathrm{PO}}_4^{3-}$