This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Esophagus, Subject name: Surgery
[{'content': 'MC site of Ca esophagus is: \nSelect and explain. \nA. Middle 1/3 rd, B. Upper 1/3 rd, C. Lower 1/3 rd, D. Lower end of esophagus', 'role': 'user'}, {'content': 'Answer: A \nExplanation: MC site of Ca esophagus is middle 1/3 rd. Squamous cell usually affects the middle one-thirds;adenocarcinoma usually affects the lower third', 'role': 'assistant'}]