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': "John Snow's discovery of cholera is an example of- \nSelect and explain. \nA. Natural experiments, B. Spot map study, C. Randomized trial, D. Sho study", 'role': 'user'}, {'content': "Answer: B \nExplanation: Ans. is 'b' i.e., Spot map study", 'role': 'assistant'}]