When $ZnS$ and $PbS$ minerals are present together,then $NaCN$ is added to separate them in the froth floatation process as a depressant,because

• A
  $Pb(CN)_2$ is precipitated while no effect on $ZnS$
• B
  $ZnS$ forms soluble complex $Na_2[Zn(CN)_4]$
• C
  $PbS$ forms soluble complex $Na_2[Pb(CN)_4]$
• D
  They cannot be separated by adding $NaCN$