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: Medicine
[{'content': 'Commonest lesion in middle mediastinum is \nSelect and explain. \nA. Lipoma, B. Aneurysm, C. Congenital cyst, D. Germ cell tumour', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]