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': 'Shade determination of composite resin restoration on a tooth should be done; \nSelect and explain. \nA. Before placement of rubber dam, B. On a dry tooth, C. Under bright light, D. Using a dry shade guide', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]