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': 'A major factor determining the efficiency of bur is \nSelect and explain. \nA. Taper angle, B. Spiral angle, C. Head length, D. Head diameter', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]