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': 'In doing gingival retraction, all of the following are used except \nSelect and explain. \nA. 8% racemic epinephrine, B. 8% alum, C. 8% zinc chloride, D. 40% zinc chloride', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]