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
The best sampling method used for drug abusers 
Select and explain. 
A. Quota sampling, B. Snowball sampling, C. Cluster random sampling, D. None of the above
B: Snowball sampling
Answer: B: Snowball sampling 
Explanation: Snow-ball Sampling

A technique for developing a research sample where existing study subjects recruit future subjects from among their acquaintances; thus the sample group appears to grow like a rolling snowball
Is often used in hidden populations which are difficult for researchers to access, e.g. drug users or commercial sex workers.