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

• A
  $3Cl_{2\text{(g)}} + 6OH^-{_{\text{(aq)}}} \rightarrow ClO_3^-{_{\text{(aq)}}} + 5Cl^-{_{\text{(aq)}}} + 3H_2O_{\text{(l)}}$
• B
  $Ag^{2+}_{(aq)} + Ag_{(s)} \longrightarrow 2 Ag^+_{(aq)}$
• C
  $Zn_{(s)} + CuSO_{4(aq)} \longrightarrow Cu_{(s)} + ZnSO_{4(aq)}$
• D
  $2 KClO_{3(s)} \longrightarrow 2 KCl_{(s)} + 3 O_{2(g)}$