NaCN is sometimes added in the froth floatation process as 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