Which one of the following is an example of a disproportionation reaction?

• A
  $3 MnO_{4}^{2-} + 4 H^{+} \rightarrow 2 MnO_{4}^{-} + MnO_{2} + 2 H_{2}O$
• B
  $MnO_{4}^{2-} + 4 H^{+} + 4 e^{-} \rightarrow MnO_{2} + 2 H_{2}O$
• C
  $10 I^{-} + 2 MnO_{4}^{-} + 16 H^{+} \rightarrow 2 Mn^{2+} + 8 H_{2}O + 5 I_{2}$
• D
  $8 MnO_{4}^{-} + 3 S_{2}O_{3}^{2-} + H_{2}O \rightarrow 8 MnO_{2} + 6 SO_{4}^{2-} + 2 OH^{-}$