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': 'Which of the following protapers have almost similar size of the tip? \nSelect and explain. \nA. S1 & Sx, B. S1 & S2, C. S2 & F1, D. S2 & L1', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]