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
Angular bone defects are classified 
Select and explain. 
A. According to the depth of the pockets, B. According to the number of walls involved, C. According to type of pockets, D. According to the number of walls remaining
D: According to the number of walls remaining
Answer: D: According to the number of walls remaining 
Explanation: None