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: Surgery
[{'content': "Jefferson's fracture is a fracture of the following veebra: \nSelect and explain. \nA. C 1, B. C 2, C. T 1, D. None of the above", 'role': 'user'}, {'content': "Answer: A \nExplanation: Jefferson fracture is a bursting fracture of the ring of C1 (the atlas) due to compression forces. There are usually two or more fractures through the ring of C1. The open-mouth odontoid view may show lateral dislocation of the lateral masses of C1. The rule of Spence states that 7 mm or greater combined dislocation indicates disruption of the transverse ligament. The transverse ligament stabilizes C1 with respect to C2. Jefferson fractures dislocated Ref: Schwaz's principle of surgery 9th edition, chapter 42.", 'role': 'assistant'}]