Which of the following is incorrect?

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

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

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

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