This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Environment and health, Subject name: Social & Preventive Medicine
Which of the following is not a indoor air pollutant? 
Select and explain. 
A. Carbon monoxide, B. Nitrous oxide, C. Radon, D. Mercury
D: Mercury
Answer: D: Mercury 
Explanation: Major air pollutants from indoor sources include carbon monoxide, polycyclic aromatic hydrocarbons, nitrogen oxides, sulphur oxides, arsenic and fluoride and radon. Park's Textbook of Preventive and Social Medicine, 25th edition, Page No. 796