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
Lt border of the heart in CXR is formed by – a) Pulmonary arteryb) Pulmonary  veinc) Abdominal aortad) Arch of aortae) Rt ventricles 
Select and explain. 
A. b, B. c, C. ad, D. ab
C: ad
Answer: C: ad 
Explanation: The left margin of heart is formed by aortic knuckle (prominence due to aortic arch), pulmonary trunk, left auricle, and left ventricle.