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': 'In Erythroblastosis fetalis, the first child is spared because: \nSelect and explain. \nA. Immune response in second exposure is fast, B. Sensitization of Rh-negative mothers by a Rh-positive fetus generally occurs at bih, C. Small amounts of fetal blood leak into the ma ternal circulation at the time of delivery, D. Mothers develop significant titers of anti-Rh ag glutinins during the postpaum period', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Sensitization of Rh-negative mothers by a Rh-positive fetus generally occurs at bih', 'role': 'assistant'}]