$51.\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_$ and $\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_$ are common impurities found in refinery feedstock, which can negatively impact refining processes. $($a$)$ Sulfur, nitrogen $($b$)$ Oxygen, mercury $($c$)$ Carbon, hydrogen $($d$)$ Methane, propane

$52.$ The production of $\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_$ and $\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_$ involves the cracking of hydrocarbons to form smaller molecules, which are then used as precursors for various petrochemicals. $($a$)$ Ethylene, propylene $($b$)$ Butylene, acetylene $($c$)$ Synthesis gas, hydrogen $($d$)$ Naphthenes, aromatics

$53.\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_$ and $\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_$ are common petrochemical derivatives used in the production of plastics, fibers, and packaging materials. $($a$)$ Polyethylene, polypropylene $($b$)$ Ethanol, methanol $($c$)$ Acetylene, butylene $($d$)$ Propane, butane

$54.$ The primary function of $\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_$ and $\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_$ in petrochemical processes is to act as intermediates in the production of various chemicals and polymers. $($a$)$ Ethylene, propylene $($b$)$ Hydrogen, synthesis gas $($c$)$ Butylene, acetylene $($d$)$ Butane, methane