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
Children surveyed in cluster sampling for coverage of national immunization programme is: 
Select and explain. 
A. 30 cluster of 5 children, B. 30 cluster of 7 children, C. 30 cluster of 10 children, D. 30 cluster of 15 children
B: 30 cluster of 7 children
Answer: B: 30 cluster of 7 children 
Explanation: 30 cluster of 7 children