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
The periodontal ligament in the region of permanent mandibular canine tends to become thinner with age, due to: 
Select and explain. 
A. 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.
A: Cementum and bone deposition.
Answer: A: Cementum and bone deposition. 
Explanation: None