During the electrolytic reduction of alumina,the reaction at the cathode is:

• A
  $2H_2O \longrightarrow O_2 + 4H^+ + 4e^-$
• B
  $3F^- \longrightarrow 3F + 3e^-$
• C
  $Al^{3+} + 3e^- \longrightarrow Al$
• D
  $2H^+ + 2e^- \longrightarrow H_2$