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 distance between the bottom of calculus and alveolar crest in human periodontal pocket is constant having an approximate length of 
Select and explain. 
A. 0.97 mm, B. 1.97 mm, C. 2.97 mm, D. 3.97 mm
B: 1.97 mm
Answer: B: 1.97 mm 
Explanation: None