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
[{'content': 'Sampling that involves randomly selecting some of the pre existing natural groups is called: \nSelect and explain. \nA. Cluster, B. Quota, C. Systematic, D. Simple random', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]