In the electrolysis of molten aluminium oxide for the production of aluminium with carbon electrodes, two reactions take place one after the other at the anode, and each one gives a different gas. What happens to the oxidation number of oxygen in these reactions?
- Adecreases by $2$, then increases by $2$
- Bincreases by $2$, then decreases by $2$
- Cincreases by $2$, then decreases by $4$
- Dno change, then decreases by $2$