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 periodontal ligament in the region of permanent mandibular canine tends to become thinner with age, due to: \nSelect and explain. \nA. Cementum and bone deposition., B. Down growths of gingival epithelium., C. Degeneration of cell rests of mallassazes., D. Horizontal and oblique fibres reversing function with age.', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]