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
The microhardness of dentin is about: 
Select and explain. 
A. 1/2 that of enamel, B. 1/7 that of enamel, C. 1/4 that of enamel, D. 1/5 that of enamel
D: 1/5 that of enamel
Answer: D: 1/5 that of enamel 
Explanation: None