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': 'All are indicators for air pollution, except – \nSelect and explain. \nA. CO2, B. SO2, C. Soiling Index, D. Smoke index', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]