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: Radiology
[{'content': 'Lt border of the heart in CXR is formed by – a) Pulmonary arteryb) Pulmonary  veinc) Abdominal aortad) Arch of aortae) Rt ventricles \nSelect and explain. \nA. b, B. c, C. ad, D. ab', 'role': 'user'}, {'content': 'Answer: C \nExplanation: The\xa0left margin of heart is formed by aortic knuckle (prominence due to aortic arch), pulmonary trunk, left auricle, and left ventricle.', 'role': 'assistant'}]