The conjugate base of phosphorus acid is $x$. The conjugate base of oleum is $y$. What are $x$ and $y$,respectively?

• A
  $H_2PO_4^{-}, H_2S_2O_7^{-}$
• B
  $H_2PO_4^{-}, HSO_5^{-}$
• C
  $H_2PO_3^{-}, HS_2O_7^{-}$
• D
  $H_2PO_3^{-}, HSO_4^{-}$