In the extraction of aluminium by the Hall-Heroult process, purified ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ is mixed with ${\mathrm{CaF}}_{2}$ to:

 a. Lower the melting point of ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ b. Increase the conductivity of the molten mixture. c. Reduce ${\mathrm{Al}}^{3+}$into $\mathrm{Al}\left(\mathrm{s}\right)$ d. Act as a catalyst

Choose the correct option:
1. a and b
2. b and c
3. c and d
4. a and d

HINT: CaF2 makes alumina a good conductor of electricity.
Explanation :
In the metallurgy of aluminium ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ is mixed with ${\mathrm{Na}}_{3}{\mathrm{AlF}}_{6}$ and ${\mathrm{CaF}}_{2}$ which causes following affects.
(i) Lower the melting point of Al, O,
(ii) Increase the conductivity of molten mixture