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': 'The denominator for calculating the incidence rate of specific disease is which of the following? \nSelect and explain. \nA. Mid year population, B. Population at risk, C. Population observed, D. Total population', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Incidence rate = (No: new cases in a given time period) / (Population at risk during that time period) x 1000', 'role': 'assistant'}]