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: Dental
[{'content': 'Back washing is a recommended technique for \nSelect and explain. \nA. Rapid Sand Filter, B. Slow Sand Filter, C. Community Well, D. Domestic Filter', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]