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
Factors which contribute to the risk of developing periodontal disease in individual teeth include 
Select and explain. 
A. Occlusion, B. Excessive occlusal stress, C. Pulpal infections, D. All of the above
D: All of the above
Answer: D: All of the above 
Explanation: None