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': 'Earliest sign of left atrial hyperophy is: \nSelect and explain. \nA. Posterior displacement of esophagus, B. Widening of carinal angle, C. Upward displacement of left bronchus, D. Double contour of right border of hea', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. Posterior displacement of esophagus', 'role': 'assistant'}]