39. (c) (i) Write the formula of cryolite.   40. (ii) Write down the word which correctly completes the following sentence: By dissolving aluminium oxide in cryolite a ................. (conducting/non-conducting) solution is produced.   41. (iii) Why is so much graphite required for this electrolytic process?   42. (iv) Write the equation for the reaction which takes place at the cathode.