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
[{'content': 'All of the following are true about cluster sampling except - \nSelect and explain. \nA. Samples are similar to those in simple Random sampling, B. Is a Rapid and simple method, C. The sample size may vary according to study design, D. It is a type of probability sample', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]