The Gibbs energy for the decomposition of ${\text{Al}}_2{\text{O}}_3$ at $500^{\circ }\text{C}$ is as follows:
$$\frac{2}{3}{\text{Al}}_2{\text{O}}_3\to \frac{4}{3}\text{Al}+{\text{O}}_2;{\Delta }_{r}G=+960{\text{kJ mol}}^{-1}$$
The potential difference needed for the electrolytic reduction of aluminium oxide (${\text{Al}}_2{\text{O}}_3$) at $500^{\circ }\text{C}$ is at least:

1. A
   $5.0\text{V}$
2. B
   $4.5\text{V}$
3. C
   $3.0\text{V}$
4. D
   $2.5\text{V}$