Which of the following statement is true?

(a) Sodium ethylxanthate acts as collector.

(b) Levigation can be used for sulphide ore to remove majority of gangue when density difference in high.

(c) Froth floatation can also be used for non-sulphide ores having sulphide impurities, and the ore is recovered by using suitable activator.

(d) During roasting, impurities such as S and As are removed so their volatile oxides $S{O}_2$ and $A{s}_2{O}_3$ respectively.