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': 'By which of the following method the incidence of a disease can be calculated? \nSelect and explain. \nA. Prospective study, B. Retrospective study, C. Case control study, D. Cross sectional study', 'role': 'user'}, {'content': "Answer: A \nExplanation: Prospective study is the exposure to the risk factor and subsequent health outcomes are observed after the beginning of the study. Incidence can be calculated by this study. Ref: Park's Textbook of Preventive and Social medicine, 19th edition, Page 71, 74.", 'role': 'assistant'}]