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
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: 
Select and explain. 
A. 1.80%, B. 2%, C. 5%, D. 20%
B: 2%
Answer: B: 2% 
Explanation: Incidence = Number of New cases x1000 Total population under risk Incidence = 90/(5000-500)X1000 = 90/4500X1000=2 Ref: Park 22nd ed /p-58.