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

• A

  $\mathrm{H}_2 \mathrm{PO}_4^{-}, \mathrm{HS}_2 \mathrm{O}_7^{-}$
• B

  $\mathrm{H}_2 \mathrm{PO}_4^{-}, \mathrm{HSO}_5^{-}$
• C

  $\mathrm{H}_2 \mathrm{PO}_3^{-}, \mathrm{HS}_2 \mathrm{O}_7^{-}$
• Correct answer
  D

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

Hence, the correct answer is option: 'D'.