This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Head and neck, Subject name: Anatomy
[{'content': 'Extentsion of neck occurs at \nSelect and explain. \nA. Atlanto-occipital joint, B. C4-C5 veebra, C. Atlantoaxial joint, D. C6-C7', 'role': 'user'}, {'content': 'Answer: A \nExplanation: C1 and C2 form a unique set of aiculations that provide a great degree of mobility for the skull. C1 serves as a ring or washer that the skull rests upon and aiculates in a pivot joint with the dens or odontoid process of C2. Approximately 50% of flexion extension of the neck happens between the occiput and C1; 50% of the rotation of the neck happens between C1 and C2. Ref - medscape.com', 'role': 'assistant'}]