<prompt> In the composition of normal air of the atmosphere at ground level in an unpolluted area, which one among the following is found inhighest concentration ?Carbon dioxideHydrogenMethaneNitrous oxide  <response>: The normal composition of external air by volume is nitrogen-78.1,oxygen-20.93, carbon dioxide- 0.03, Argon 0.9. Parks Textbook of Preventive and Social Medicine, 25th Edition, Pg 791 