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': 'Commonest cause of ectopic pregnancy is : \nSelect and explain. \nA. Endometriosis, B. Previous salpingitis, C. Anemia, D. Cervicitis', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Previous salpingitis', 'role': 'assistant'}]