Which of the following reactions occurs at the anode when the electrolysis of an aqueous $CuSO_4$ solution is carried out using copper electrodes?

• A
  $Cu \rightarrow Cu^{2+} + 2e^-$
• B
  $2SO_4^{2-} + 2H_2O \rightarrow 2H_2SO_4 + O_2 + 4e^-$
• C
  $2H_2O \rightarrow O_2 + 4H^{+} + 4e^-$
• D
  $2Cu \rightarrow Cu_2^{2+} + 2e^-$