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 mantle dentin found alon DEJ is 
Select and explain. 
A. 20 μm, B. 30 μm, C. 40 μm, D. 50 μm
A: 20 μm
Answer: A: 20 μm 
Explanation: The mantle dentin found along the DEJ is about 20μm thick and it contains larger-diameter argyrophilic (silver stained) collagen fibers arranged perpendicularly to the DEJ. The circumpulpal dentin contains smaller-diameter closely packed collagen fibers. The dentin which forms after root completion is called secondary dentin.