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
Over denture used to 
Select and explain. 
A. Preserve alveolar bone, B. Maintain proprioception from periodontal ligament, C. Retention, D. All of the above
D: All of the above
Answer: D: All of the above 
Explanation: None