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': 'Ferning of cervical mucus disappears after -day of menstrual cycle \nSelect and explain. \nA. 7th, B. 15th, C. 18th, D. 21st', 'role': 'user'}, {'content': 'Answer: D \nExplanation: 21st', 'role': 'assistant'}]