This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Infectious Disease Epidemiology, Subject name: Social & Preventive Medicine
[{'content': 'Serial interval is: \nSelect and explain. \nA. Gap between primary and secondary cases, B. Gap between maximum and minimum incubation period, C. Difference between sensitivity and specificity, D. Indicates secondary attack rate.', 'role': 'user'}, {'content': "Answer: A \nExplanation: Ans. is 'a' i.e. Gap between primary & secondary cases", 'role': 'assistant'}]