$66.\_\_\_,\_\_\_,$ and $\_\_\_$ are common methods employed in refinery operations to convert heavier hydrocarbons into lighter, more valuable products. $($a$)$ Catalytic cracking, hydrocracking, reforming $($b$)$ Distillation, desulfurization, denitrification $($c$)$ Polymerization, hydrogenation, isomerization $($d$)$ Steam cracking, alkylation, dehydration