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 microhardness of dentin is about: \nSelect and explain. \nA. 1/2 that of enamel, B. 1/7 that of enamel, C. 1/4 that of enamel, D. 1/5 that of enamel', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]