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
\r\nThe denominator for calculating the incidence rate of specific disease is which of the following? 
Select and explain. 
A. Mid year population, B. Population at risk, C. Population observed, D. Total population
B: Population at risk
Answer: B: Population at risk 
Explanation: Incidence rate = (No: new cases in a given time period) / (Population at risk during that time period) x 1000