$61.\_\_\_,\_\_\_,$ and $\_\_\_$ are common impurities found in refinery feedstock, which can adversely affect the efficiency and lifespan of refining equipment. $($a$)$ Sulfur, nitrogen, oxygen $($b$)$ Carbon, hydrogen, methane $($c$)$ Mercury, arsenic, lead $($d$)$ Butane, propane, ethylene

$66.\_\_\_,\_\_\_,$ 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