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': 'Randomized controlled trials are all except: \nSelect and explain. \nA. Clinical trials., B. Preventive trials., C. Before and after comparison studies., D. Evaluation of health services.', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]