Which of the following is NOT a common component of vehicle exhaust?

The choice of oxygen as the correct answer highlights that it is not typically found in significant quantities as a component of vehicle exhaust. In the context of vehicle emissions, hydrocarbons, carbon monoxide, and nitrogen oxides are common byproducts of fuel combustion.

Hydrocarbons are organic compounds that can exist in the exhaust because they originate from unburned fuel and engine operation. Carbon monoxide results from incomplete combustion and is a toxic gas produced when fuel does not burn completely. Nitrogen oxides are produced from the reaction between nitrogen in the air and oxygen at the high temperatures of combustion and contribute to smog and acid rain.

While oxygen is present in the atmosphere and is used in the combustion process, it is not a significant component of the exhaust gases emitted from vehicles. After combustion, the oxygen content in the exhaust is generally low because most of it is consumed during the combustion process. This distinction is what makes oxygen the correct choice as the component that is not commonly found in vehicle exhaust.