Lime stone is added in the extraction of lead from galena why?

The correct option is D All of the above
$CaO+Si{O}_2\to CaSi{O}_3\left(Slag\right)$
$PbO+Si{O}_2\to PbSi{O}_3$
$CaO$ converts $PbSi{O}_3$ to $PbO$,
$PbSi{O}_3+CaO\to PbO+CaSi{O}_3$
and also prevents the formation of $PbS{O}_4$