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 instrument that has four unit formula \nSelect and explain. \nA. Gingival margin trimmer, B. Enamel hatchet, C. Hoe, D. Chisel', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]