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': 'Incidence can be calculated by - \nSelect and explain. \nA. Prospective study, B. Retrospective study, C. Case control study, D. Cross-sectional study', 'role': 'user'}, {'content': "Answer: A \nExplanation: Ans. is 'a' i.e., Prospective study", 'role': 'assistant'}]