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
How much guttapercha should be left to prevent leakage for post placement? 
Select and explain. 
A. 4mm, B. 2mm, C. 1mm, D. 3mm
A: 4mm
Answer: A: 4mm 
Explanation: None