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 number of walls in the apical portion of the defect is often greater than that of its occlusal portion, is referred to as \nSelect and explain. \nA. Osseous Craters, B. Combined osseous defect, C. Horizontal defect, D. Vertical defect', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]