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
[{'content': 'All of the following can be used for root biomodification except: \nSelect and explain. \nA. Chlorhexidine, B. Citric acid, C. Tetracycline, D. HCL', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]