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
[{'content': 'How much guttapercha should be left to prevent leakage for post placement? \nSelect and explain. \nA. 4mm, B. 2mm, C. 1mm, D. 3mm', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]