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': 'Periodontal ligament is thinnest at \nSelect and explain. \nA. Apex, B. Coronal, C. Crestal, D. Fulcrum of axis of rotation', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]