1) as we know that roasting is done in excess of oxygen. this is done because when heated in excess oxygen their impurities of sulphur and arsenic are removed as volatile oxides and sulphide ore is converted into its oxides.
3) okay m going to give an example like if you have to defeat a car in racing then it's obvious that you have to buy a car faster than that car.
same case is with the elements of in reactivity series. now you can't extract aluminium with metals below it in reactivity series.
you can only do it by elements like potassium, sodium, calcium because that are more powerful and reactive than aluminium.