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
All of the following can be used for root biomodification except: 
Select and explain. 
A. Chlorhexidine, B. Citric acid, C. Tetracycline, D. HCL
A: Chlorhexidine
Answer: A: Chlorhexidine 
Explanation: None