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
What is the thickness of the layer of prismless enamel found in primary teeth? 
Select and explain. 
A. 25 μm, B. 50 μm, C. 75 μm, D. 100 μm
A: 25 μm
Answer: A: 25 μm 
Explanation: None