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': 'MC cause of secondary PPH is: \nSelect and explain. \nA. Atonic uterus, B. Retained placenta, C. latrogenic injury, D. Medical diseases', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Retained placenta', 'role': 'assistant'}]