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': 'Primary and secondary case are used to define? \nSelect and explain. \nA. Generation time, B. Serial interval, C. Secondary attack rate, D. None', 'role': 'user'}, {'content': "Answer: B \nExplanation: Ans. is 'b' i.e., Serial interval", 'role': 'assistant'}]