This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Epidemiology, Subject name: Social & Preventive Medicine
[{'content': 'Following is true about longitudinal studies \nSelect and explain. \nA. Used to study the natural history of the disease, B. Single outcome, C. Are economical, D. Are efficient', 'role': 'user'}, {'content': "Answer: A \nExplanation: Longitudinal studies are useful in: To study the natural history of disease and it's future outcome For identifying risk factors of disease For finding out incidence rate or rate of occurrence of new cases of disease in the community Ref: 25th edition, Park's Textbook of Preventive and Social Medicine, Page no. 78", 'role': 'assistant'}]