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. Clinical trials, B. Preventive trials, C. Before and after comparison studies, D. Evaluation of Health Services', 'role': 'user'}, {'content': "Answer: A \nExplanation: Ans. is 'a' i.e., Clincial trials", 'role': 'assistant'}]