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
Serial interval is: 
Select and explain. 
A. 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.
A: Gap between primary and secondary cases
Answer: A: Gap between primary and secondary cases 
Explanation: Ans. is 'a' i.e. Gap between primary & secondary cases