This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Epidemiology, Subject name: Social & Preventive Medicine
[{'content': 'Most important indicator of air pollution is: \nSelect and explain. \nA. SO2, B. CO2, C. CO2, D. N2O', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. a.SO2 Chemical indicator of air pollution:Sulfur Dioxide: Best indicator of Air Pollution.Smoke or soiling index: Air strain in a filter paper measured through photoelectric meter.Grit and dust measurementAir pollution indexSoiling indexCoefficient of hazeBiological indicator of air pollution: Lichens', 'role': 'assistant'}]