Which of the following reactions occurs at the cathode of a common dry cell?

• A
  $Mn \to Mn^{2+} + 2e^-$
• B
  $2MnO_2 + Zn^{2+} + 2e^- \to ZnMn_2O_4$
• C
  $2ZnO_2 + Mn^{2+} + 2e^- \to MnZn_2O_4$
• D
  $Zn \to Zn^{2+} + 2e^-$