This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Carcinoma Ovary, Subject name: Gynaecology & Obstetrics
[{'content': 'Protective factor for ovarian cancer: \nSelect and explain. \nA. Nulliparity, B. Bilateral Tubectomy, C. Dysgenetic gonads, D. Woker of asbestos industry', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Ans is b, i.e. Bilateral tubectomyRef: William gynae 3/ed, p736"Tubal ligation and Hysterectomy are each associated with a substantial reduction in risk. Theoretically, any gynecological procedure that precludes intants from reaching the ovaries via as caesarian from the lower genital tract might plausibly exert a similar protective effect." - Ref: Williams Gynae 3rd/ed, p736', 'role': 'assistant'}]