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
Not a constituent of air - 
Select and explain. 
A. H2, B. N2, C. O2, D. CO2
A: H2
Answer: A: H2 
Explanation: Air is a mechanical mixture of gases. It constitutes nitrogen 78.1%, oxygen 20.93%, carbon dioxide0.03%, argon, neon, krypton, xenon and helium. In addition to these gases, air also contains water vapour, traces of ammonia and suspended matter. Park's Textbook of Preventive and Social Medicine, 25th edition, Page No.791