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': 'Case control study is an example of ? \nSelect and explain. \nA. Prospective study, B. Retrospective study, C. Combined retrospective and prospective study, D. Study at one point of time', 'role': 'user'}, {'content': "Answer: B \nExplanation: Ans. is 'b' i.e., Retrospective study", 'role': 'assistant'}]