This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Biostatistics, Subject name: Social & Preventive Medicine
[{'content': 'True statements with regard to sampling \nSelect and explain. \nA. Snowball sampling is used for hidden population, B. More sample in systematic random sampling, C. In stratified random sampling population is divided into strata, D. Cluster sampling is less cost - effective', 'role': 'user'}, {'content': 'Answer: A \nExplanation: .', 'role': 'assistant'}]