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': 'In a population of 5000 people, 500 are already myopic on January 1, 2011, number of new myopic cases till December 31, 2011 is 9. Calculate incidence: \nSelect and explain. \nA. 1.80%, B. 2%, C. 5%, D. 20%', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Incidence = Number of New cases x1000 Total population under risk Incidence = 90/(5000-500)X1000 = 90/4500X1000=2 Ref: Park 22nd ed /p-58.', 'role': 'assistant'}]