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 can be classified as following, except: \nSelect and explain. \nA. Substernal., B. Plunging., C. Nodular., D. Intra-thoracic.', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]