Which of the following is $NOT$ an anode reaction?

• A
  $Cl^{-} \to \frac{1}{2}Cl_{2} + e^{-}$
• B
  $Cu \to Cu^{2+} + 2e^{-}$
• C
  $Hg^{+} \to Hg^{2+} + e^{-}$
• D
  $Zn^{2+} + 2e^{-} \to Zn$