Which of the following is incorrect?

• A

  ${\mathrm{ClO}}_4^{-}$ has $3\mathrm{d\pi }-\mathrm{p\pi }$ bonds 

• B

  ${\mathrm{ClO}}^{-}$ is strong conjugate base 

• C

  ${\mathrm{Cl}}_2{\mathrm{O}}_7$ is most acidic oxide

• D

  ${\text{ClO}}_3^{-}\hspace{0.17em}\mathrm{and}\hspace{0.17em}{\mathrm{NO}}_3^{-}$ are isostructural