A concentrated aqueous copper(II) sulfate solution is electrolysed using copper electrodes. Which ionic half-equation shows the reaction occurring at the cathode?
- A$2\text{H}^+ + 2e^- \rightarrow \text{H}_2$
- B$4\text{OH}^- \rightarrow \text{O}_2 + 2\text{H}_2\text{O} + 4e^-$
- C$\text{Cu} \rightarrow \text{Cu}^{2+} + 2e^-$
- D$\text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}$