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': 'Estimation of percentage of children immunized in community as per WHO is to be done by: \nSelect and explain. \nA. Multistage random sampling, B. Cluster random, C. Systematic random sampling, D. All of the above', 'role': 'user'}, {'content': 'Answer: B \nExplanation: WHO recommends cluster random sampling (CRS) to estimate the percentage of children immunized in the community, paicularly in developing nations. Their recommended strategy is 30x7 sampling. Ref: Medical biostatistics, 1st edition pg: 201', 'role': 'assistant'}]