This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Epidemiology, Subject name: Social & Preventive Medicine
[{'content': 'Denominator in Incidence rate is? \nSelect and explain. \nA. Total observed population, B. Total population at risk in a year, C. Total live births, D. Total new cases in a year', 'role': 'user'}, {'content': "Answer: B \nExplanation: ANSWER: (B) Total population at risk in a yearREF: Park's 22 Ed page 58Indirect repeat Community medicine Post diploma June 2012", 'role': 'assistant'}]