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
Instrumentation zone in periodontal therapy is: 
Select and explain. 
A. The portion of the tooth where calculus (or) necrotic cementum is found, B. C.E. Junction, C. Exposed root surfaces, D. Deep periodontal pocket
A: The portion of the tooth where calculus (or) necrotic cementum is found
Answer: A: The portion of the tooth where calculus (or) necrotic cementum is found 
Explanation: None