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: Medicine
[{'content': 'Which is least common site of berry aneurysm - \nSelect and explain. \nA. Basilar artery, B. Vertebral artery, C. Anterior cerebral artery, D. Posterior cerebral artery', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]