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': 'The commonest type of ovarian tumour is : \nSelect and explain. \nA. Dermoids, B. Cystadenoma, C. Fibroma, D. Cystadenocarcinoma', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Cystadenoma', 'role': 'assistant'}]