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
[{'content': 'Not a constituent of air - \nSelect and explain. \nA. H2, B. N2, C. O2, D. CO2', 'role': 'user'}, {'content': "Answer: A \nExplanation: 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", 'role': 'assistant'}]