This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Epidemiology, Subject name: Social & Preventive Medicine
[{'content': 'All are example for randomised 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: .the major type of randomised controlled trials includes, 1.clinical trials 2.preventive trials. 3.risk factor trials 4.cessation experimnts 5.trial of aetiological agents. 6.evaluation of health services. ref:park&;s textbook,ed 22,pg no 82', 'role': 'assistant'}]