Not a constituent of air -: (A) H2, (B) N2, (C) O2, (D) CO2
Answer is A. 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