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: Dental
[{'content': 'Case control study : \nSelect and explain. \nA. Is a prospective study, B. Gives Odd ratio, C. Useful in case of rare disease, D. Both 2 and 3', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Case control:\n\nA retrospective study.\nRisk factor is to be identified — disease present \nGives\xa0Odds\xa0ratio.\nBest method to study RARE disease — Case control study.\n\nNESTED Case-control study\n\nIt is a case-control study nested within COHORT study. So, it is a prospective\xa0study.', 'role': 'assistant'}]