Which one of the following reactions occurs at the anode in the Castner process of extracting sodium metal?

• A
  $H_2 \longrightarrow 2 H^{+} + 2 e^{-}$
• B
  $2 Cl^{-} \longrightarrow Cl_2 + 2 e^{-}$
• C
  $4 OH^{-} \longrightarrow 2 H_2 O + O_2 + 4 e^{-}$
• D
  $Na^{+} + e^{-} \longrightarrow Na$