$21.$ Fuel oil is primarily used in:

a$)$ Gasoline engines

b$)$ Diesel engines

c$)$ Aircraft engines

d$)$ Steam engines

$22.$ Lubricants are used to:

a$)$ Increase friction between moving parts

b$)$ Decrease wear and tear of machinery

c$)$ Clean engine components

d$)$ Improve fuel efficiency

$23.$ Petroleum waxes are commonly used in:

a$)$ Candle making

b$)$ Soap production

c$)$ Plastic manufacturing

d$)$ Paint production

$24.$ Petroleum coke is a by$-$product of:

a$)$ Refining

b$)$ Combustion

c$)$ Cracking

d$)$ Polymerization