During which of the following changes will an exchange of $5$ electrons take place?

• A
  $MnO_4^- \to Mn^{2+}$
• B
  $CrO_4^{2-} \to Cr^{3+}$
• C
  $MnO_4^{2-} \to MnO_2$
• D
  $Cr_2O_7^{2-} \to 2Cr^{3+}$