Which of the following is a disproportionation reaction?

• A
  $Cu_2O + 2H^{+} \to Cu + Cu^{2+} + H_2O$
• B
  $2CrO_4^{2-} + 2H^{+} \to Cr_2O_7^{2-} + H_2O$
• C
  $CaCO_3 + 2H^{+} \to Ca^{2+} + H_2O + CO_2$
• D
  $Cr_2O_7^{2-} + 2OH^{-} \to 2Cr_2O_4^{2-} + H_2O$