Answer the following question given this paragraph: 

Catalytic converters in cars increase the rates of several important reactions. Unburned hydrocarbons can more easily be converted to carbon dioxide and water in the presence of a catalyst. Additionally, carbon monoxide (a harmful gas produced by incomplete combustion of fuel) is further oxidized to carbon dioxide, and nitrous oxide compounds (which contribute to smog and acid rain) are transformed back into nitrogen and oxygen gases. By passing car exhaust over these catalysts before their release into the air, the emissions from a combustion engine are made into much less harmful substances, resulting in a much cleaner atmosphere.


Q: Unburned hydrocarbons can more easily be converted to carbon dioxide and water in the presence of a what?

 Choices:

- temperature

- methane

- Ethanol

- catalyst

A:
catalyst
