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
True statements with regard to sampling-a) Snowball sampling is used for hidden populationb) More sample in systemic random samplingc) In stratified random sampling population is divided into stratad) Cluster sampling is less cost-effective 
Select and explain. 
A. ac, B. bc, C. ad, D. b
A: ac
Answer: A: ac 
Explanation: None