[" The function of fluorspar in the electrolytic reduction of alumina dissolved in fused cryol: "],[(Na_(3)AlF_(6))" is: "],[" (a) as a catalyst "],[" (b) to lower the temperature of the melt and to make the fused mixture very conducting "],[" (c) to decrease the rate of oxidation of carbon at the anode "],[" (d) none of the above "]