This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Aerial disorders, Subject name: Surgery
[{'content': 'Commonest pa of subclan aery to be affected by Stenosis is: \nSelect and explain. \nA. First pa, B. Second pa, C. Third pa, D. Equally affected', 'role': 'user'}, {'content': 'Answer: A \nExplanation: MC site of subclan aery Stenosis is the first pa of the subclan aery.', 'role': 'assistant'}]