Which of the following is an unlikely source of carbon monoxide? 

A)  cigarette smoking 
B)  faulty heating systems 
C)  anaerobic respiration 
D)  motor vehicles

Carbon Monoxide

A gas that is clear, has no smell, and is poisonous, resulting from the incomplete burning of materials containing carbon.

Anaerobic Respiration

is a process by which cells generate energy in the absence of oxygen, using other electron acceptors to metabolize substrates.

Cigarette Smoking

The act of inhaling smoke from burning tobacco wrapped in paper; a leading cause of numerous health issues, including lung cancer.