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': 'Best approach to study a rare disease is: \nSelect and explain. \nA. Case control study, B. Cross-sectional study, C. Cohort study, D. Clinical trial', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]