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': 'Venturimeter is used to measure \nSelect and explain. \nA. Air velocity, B. Size of suspended particles in the air, C. Bed resistance in a slow sand filter, D. SO2 content in the atmosphere', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]