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 statements regarding progesterone only pill are true, except \nSelect and explain. \nA. Inhibits ovulation, B. Failure rate is the same as combined OCP, C. Irregular bleeding is a known complication, D. Acts by altering the cervical mucous', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Failure rate of combined OCP is 0.2/100 women years, whereas failure rate of progesterone only pill is about 0.5 - 2 per 100 women years of use. Progesterone only pill acts by thickening cervical mucous, causing atrophy of endometrium or by inhibiting ovulation.', 'role': 'assistant'}]