This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Environment and health, Subject name: Social & Preventive Medicine
[{'content': 'Soiling index is a measure of \nSelect and explain. \nA. Soil pollution, B. Water pollution, C. Noise pollution, D. Air pollution', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Smoke or soiling index: A known volume of air is filtered through a white paper under specified conditions and the stain is measured by the photoelectric meter.Smoke concentration is estimated and expressed as micrograms/cubic metre of air as an average level over a period of time.Park 23e pg: 736', 'role': 'assistant'}]