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: Gynaecology & Obstetrics
[{'content': 'All of the following ovarian tumours usually occur bilaterally, except: \nSelect and explain. \nA. Metastatic mass, B. Dysgerminoma, C. Cyst adenoma of ovary, D. Dermoid cyst', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]