Which of the following is not an example of an acid buffer?

• A

  ${\mathrm{H}}_3{\mathrm{PO}}_4+{\mathrm{H}}_2{\mathrm{PO}}_4^{-}$

• B

  ${\mathrm{H}}_2{\mathrm{PO}}_4^{-}+{\mathrm{HPO}}_4^{2-}$

• C

  ${\mathrm{H}}_2{\mathrm{C}}_2{\mathrm{O}}_4+{\mathrm{Na}}_2{\mathrm{C}}_2{\mathrm{O}}_4$

• D

  ${\mathrm{HNO}}_3+{\mathrm{NaNO}}_3$