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': 'Functions of periodontal ligament \nSelect and explain. \nA. Nutritive, B. Sensory, C. Formative, D. All', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]