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': 'All of the following may result in under filling of a root canal EXCEPT \nSelect and explain. \nA. Apical perforation, B. Edge formation, C. Short length estimation, D. Lateral perforation', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]