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
[{'content': '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 ? \nSelect and explain. \nA. Carbon dioxide, B. Hydrogen, C. Methane, D. Nitrous oxide', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]