NaCN is sometimes added in the forth flotation process as a depressant when ZnS and PbS minerals are expected because :
(A) Pb(CN)2 is precipitated while no effect on ZnS.
(B) ZnS forms soluble complex Na2[Zn(CN)4] while PbS forms froth
(C) PbS forms soluble complex Na2[Pb(CN)4] while ZnS forms froth.
(D) NaCN is never added in froth floatation process.