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': 'In maternal and child welfare program the sampling is done by which method? \nSelect and explain. \nA. Systematic, B. Cluster-30, C. Group, D. Stratified', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]