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': 'Which is the most common agent employed in external bleaching? \nSelect and explain. \nA. Sodium perborate, B. Hydrochloric acid, C. Carbamide peroxide, D. Sodium hypochlorite', 'role': 'user'}, {'content': 'Answer: C \nExplanation: The most common bleaching agent for at-home bleaching is 10% to\xa022%\xa0carbamide peroxide\xa0with\xa0an\xa0effective\xa0yield\xa0of\xa04%\xa0to 7.5% hydrogen peroxide.\nRef: Cohen pathways of Pulp 11th ed page no\xa0 e107', 'role': 'assistant'}]