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
People are separated into groups, from each group people are selected randomly. What type of sampling is this - 
Select and explain. 
A. Simple random, B. Stratified random, C. Systemic random, D. Cluster
B: Stratified random
Answer: B: Stratified random 
Explanation: None