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
A herd immunity of over .........is considered necessary to prevent epidemic spread of diphtheria: 
Select and explain. 
A. 50%, B. 55%, C. 60%, D. 70%
D: 70%
Answer: D: 70% 
Explanation: 70%