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': 'Common cause of retained placenta : \nSelect and explain. \nA. Atonic uterus, B. Constriction ring, C. Placenta accreta, D. Poor voluntary expulsive effo', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Atonic uterus', 'role': 'assistant'}]