The electrolytic reduction of alumina to aluminum is made possible by the Hall-$H$éroult process in the presence of:

• A
  In the presence of $NaCl$
• B
  In the presence of fluoride
• C
  In the presence of cryolite which lowers the melting point of the mixture
• D
  In the presence of cryolite which raises the melting point of the mixture