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: Dental
[{'content': 'The best sampling method used for drug abusers \nSelect and explain. \nA. Quota sampling, B. Snowball sampling, C. Cluster random sampling, D. None of the above', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Snow-ball Sampling\n\nA technique for developing a research sample where existing study subjects\xa0recruit future subjects from among their acquaintances; thus the sample group\xa0appears to grow like a rolling snowball\nIs often used in hidden populations which are difficult for researchers to access,\xa0e.g. drug users or commercial sex workers.', 'role': 'assistant'}]