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': 'One of the following is an example of painless midline swelling - \nSelect and explain. \nA. Branchial cyst, B. Thyroglossal cyst, C. Cystic hygroma, D. Carotid body tumour', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]