Which one of the following reactions does not involve either oxidation or reduction?

• A

  ${\text{MnO}}_{\text{4}}^{-}\to {\text{MnO}}_{\text{2}}$
• B

  $\mathrm{N}\mathrm{a}\mathrm{}\mathrm{}\to \mathrm{}\mathrm{}\mathrm{N}{\mathrm{a}}^{+}$
• Correct answer
  C

  $\mathrm{C}\mathrm{r}{\mathrm{O}}_4^{2-}\mathrm{}\mathrm{}\to \mathrm{}\mathrm{}\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7^{2-}$
• D

  $\mathrm{Z}{\mathrm{n}}^{2+}\mathrm{}\mathrm{}\to \mathrm{}\mathrm{}\mathrm{Z}\mathrm{n}$

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