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': 'On barium swallow posterior indentation is seen due to: \nSelect and explain. \nA. Left atrium, B. Aoic knuckle, C. Aberrant right subclan, D. Sling of pulmonary aery', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Ans. Aberrant right subclan', 'role': 'assistant'}]