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': 'Feature of slow sand filter \nSelect and explain. \nA. Fit in a little space, B. Filter is cleaned by back washing, C. Can be handled by less killed person, D. Used in Big cities', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]