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': 'For calculation of incidence denominator is taken as- \nSelect and explain. \nA. Mid year population, B. Population at risk, C. Total number of cases, D. Total number of deaths', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Denominator for calculating incidence is population at risk.', 'role': 'assistant'}]