This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General anatomy, Subject name: Anatomy
[{'content': 'Which of the following are unpaired vessel in CNS? \nSelect and explain. \nA. Anterior-cerebral aery, B. Basilar aery, C. Posterior- Cerebellar aery, D. Posterior-communicating aery', 'role': 'user'}, {'content': "Answer: B \nExplanation: Basilar aery is single unpaired vessel, formed by right &left veebral aeries at lower border of pons, Ref: Gray's40e/p-262", 'role': 'assistant'}]