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': 'The pH of a freshly prepared stannous fluoride solution is approximately \nSelect and explain. \nA. 2.5, B. 4.5, C. 6.5, D. 8.5', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]