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': 'The brilliantly transilluminant tumour in the neck may be- \nSelect and explain. \nA. Branchial cyst, B. Thyroglossal cyst, C. Sternomastoid tumour, D. Cystic hygroma', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]