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': 'Re-etching saliva contaminated etched enamel should be done for \nSelect and explain. \nA. 30 seconds, B. 20 seconds, C. 10 seconds, D. 5 seconds', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]