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
[{'content': 'Children surveyed in cluster sampling for coverage of national immunization programme is: \nSelect and explain. \nA. 30 cluster of 5 children, B. 30 cluster of 7 children, C. 30 cluster of 10 children, D. 30 cluster of 15 children', 'role': 'user'}, {'content': 'Answer: B \nExplanation: 30 cluster of 7 children', 'role': 'assistant'}]