Cryolite is:
A. `Na_(3)AlF_(6)` and is used in the electrolysis of aluminia for decreasing electrical conductivity.
B. `Na_(3)AlF_(6)` and is used in the electrolysis of laumina for loweing the meting point of alumina.
C. `Na_(3)AlF_(6)` and is used in the elctrolytic purification of aluminia.
D. `Na_(3)AlF_(6)` and is used in the electrolysis of alumina for increasing the melting point and electrical conductivity.