Aluminium is manufactured from aluminium oxide by electrolysis. Which row shows the ionic half-equations at each electrode and describes the role of cryolite in the process?

	reaction at anode	reaction at cathode	role of cryolite
A	2O $^{2 -}$ $\to$ O $_{2}$ + 4e $^{-}$	Al $^{3 +}$ + 3e $^{-}$ $\to$ 3Al	catalyst
B	Al $^{3 +}$ + 3e $^{-}$ $\to$ Al	2O $^{2 -}$ $\to$ O $_{2}$ + 4e $^{-}$	solvent for aluminium oxide
C	2O $^{2 -}$ $\to$ O $_{2}$ + 4e $^{-}$	Al $^{3 +}$ + 3e $^{-}$ $\to$ Al	solvent for aluminium oxide
D	Al $^{3 +}$ + 3e $^{-}$ $\to$ 3Al	2O $^{2 -}$ $\to$ O $_{2}$ + 4e $^{-}$	catalyst