This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Social & Preventive Medicine
In the composition of normal dry air of the atmosphere at ground level in an unpolluted area, which one among the following is found in highest concentration ? 
Select and explain. 
A. Carbon dioxide, B. Hydrogen, C. Methane, D. Nitrous oxide
A: Carbon dioxide
Answer: A: Carbon dioxide 
Explanation: None