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': 'Commonest site of aneurysm is - \nSelect and explain. \nA. Middle cerebral artery, B. Vertebrobasilar system, C. Anterior communicating artery, D. Internal carotid artery', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]