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: Surgery
[{'content': 'Retrosternal goiter: \nSelect and explain. \nA. Causes stridor, B. Obtains its blood supply from thoracic vessels, C. Characteristically malignant, D. Must be removed by thoracotomy', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]