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': 'Randomized controlled trials are: \nSelect and explain. \nA. Preventive trials, B. Before and after comparison studies, C. Evaluation of Health Services, D. Clinical trials', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Clinical trials', 'role': 'assistant'}]