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': 'Best indicators of the pollution \nSelect and explain. \nA. Soiling Index, B. Air pollution Index, C. Sulphur dioxide, D. Breteau Index', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]