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. Short study", 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]