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': 'Narrowing of periodontal ligament in labially placed mandibular canine with age is due to: \nSelect and explain. \nA. Due to deposition of cementum and bone, B. Down growths of gingival epithelium, C. Gingival cysts formed from cell rests, D. Reversal of function of horizontal and oblique fibres', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]