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': 'Retainer of choice in mandibular posterior teeth \nSelect and explain. \nA. Full veneer crown, B. Partial veneer crown, C. 3/4 crown, D. 7/8 crown', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]