This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Musculoskeletal system, Subject name: Anatomy
[{'content': 'Struma ovarii is composed of \nSelect and explain. \nA. Mature thyroid tissue, B. Immature thyroid tissue, C. Primary ovarian Carcionoid tissue, D. None', 'role': 'user'}, {'content': 'Answer: A \nExplanation: It is a special teratoma which is always unilateral and is composed of mature thyroid tissue', 'role': 'assistant'}]