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': 'Greater cutting efficiency of a bur is seen when the rake angle is: \nSelect and explain. \nA. Positive, B. Negative, C. Zero, D. Any of the above', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]