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
For a survey, some schools were selected randomly from which classes were selected randomly and then sections were selected randomly and from which students were selected randomly, which type of sampling method is this 
Select and explain. 
A. Simple random, B. Cluster sampling, C. Stratified sampling, D. Multistage sampling
D: Multistage sampling
Answer: D: Multistage sampling 
Explanation: None