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': 'At what level of b-HCG is it that normal pregnancy can be earliest detected by TVS : \nSelect and explain. \nA. 500 IU/m1, B. 1000 IU/ml, C. 1500 IU/ml, D. 2000 IU/ml', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]