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': 'Most common cause of failure of class II restoration in\nchildren is: \nSelect and explain. \nA. Margin failure, B. Proximal box fracture, C. Inadequate width, D. Too much width', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]