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
Main aim of root planning is to remove: 
Select and explain. 
A. Plaque, B. Calculus, C. Necrotic cementum, D. All of the above
C: Necrotic cementum
Answer: C: Necrotic cementum 
Explanation: None