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
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 ? 
Select and explain. 
A. Carbon dioxide, B. Hydrogen, C. Methane, D. Nitrous oxide
A: Carbon dioxide
Answer: A: Carbon dioxide 
Explanation: The normal composition of external air by volume is nitrogen-78.1%,oxygen-20.93%, carbon dioxide- 0.03%, Argon 0.9%. Park's Textbook of Preventive and Social Medicine, 25th Edition, Pg 791