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 prepared cavity is best protected from moisture by \nSelect and explain. \nA. Cotton rolls, B. Saliva ejector, C. Rubber dam, D. Paper napkin', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]