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': 'All are example for randomized control trials except - \nSelect and explain. \nA. Natural experiments, B. Clinical trials, C. Risk factor trials, D. Preventive trials', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]