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
Measles epidemic can be anticipated when proportion of suceptible children in a community 
Select and explain. 
A. 20%, B. 40%, C. 60%, D. 80%
B: 40%
Answer: B: 40% 
Explanation: None