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
Secondary attack rate of chickenpox ? 
Select and explain. 
A. 70%, B. 90%, C. 65%, D. 80%
B: 90%
Answer: B: 90% 
Explanation: Ans. is `b' i.e., 9%