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': 'For GV Black class V cavities, the retainer of choice is \nSelect and explain. \nA. No. 212, B. No. 112, C. No. 132, D. No. 221', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]