Which of the following reactions does not involve change in the oxidation state of the metal ?

Question

( A ) $CrO_4^{2-} \to Cr_2O_7^{2-}$
( B ) $Na \to Na^+$
( C ) $Zn^{2+} \to Zn$
( D ) $VO^{2+} \to V_2O_3$