This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Obstetrics and gynaecology, Subject name: Radiology
[{'content': 'The characteristic finding in USG of ectopic pregnancy is \nSelect and explain. \nA. Resistance in color doppler, B. Absence of gestational sac in uterus, C. Free fluid in peritoneal cavity, D. Complex adnexal mass', 'role': 'user'}, {'content': 'Answer: B \nExplanation: <img alt="" src=" />An ectopic pregnancy occurs when a feilized egg implants and grows outside the main cavity of the uterus', 'role': 'assistant'}]