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': 'The most common cause of fracture at the isthmus of a class II dental amalgam restoration: \nSelect and explain. \nA. Delayed expansion, B. Inadequate depth at the isthmus area, C. Inadequate width at the isthmus area, D. Moisture contamination of the amalgam', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]