This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Epidemiology, Subject name: Social & Preventive Medicine
Children surveyed in cluster sampling for coverage of national immunization programme in? 
Select and explain. 
A. 30 clusters of 5 children, B. 20 clusters of 5 children, C. 30 clusters of 10 children, D. 30 clusters of 7 children
D: 30 clusters of 7 children
Answer: D: 30 clusters of 7 children 
Explanation: .