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 lower blood mercury level at which the earliest non specific symptom starts appearing at: \nSelect and explain. \nA. 25 ng/ml, B. 35 ng/ml, C. 40 ng/ml, D. 45 ng/ml', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]