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': 'The terminal end of a retentive arm is optimally placed\nat \nSelect and explain. \nA. Middle of the gingival third of the clinical crown, B. Middle of the middle third of the clinical crown, C. Under the gingival sulcus in the clean area, D. Middle of occlusal third of the clinical crown', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]