Aluminium and copper are extracted from their oxide and sulphide ores respectively. 

Which of the following is correct?

I. Copper is extracted by the auto reduction of copper oxide by copper sulphide. 

II. Aluminium cannot be obtained by chemical reduction due to its strong affinity for oxygen. 

III. In electrometallurgy of Al, graphite is used as cathode to avoid reoxidation of Al into $$\text{Al}_2\text{O}_3$$ by preventing formation of $$\text{O}_2$$. 

IV. Sulphide ores of copper are difficult to be reduced than the oxide ores