This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Diagnosis in Obstetrics, Subject name: Gynaecology & Obstetrics
[{'content': 'At what level of b-hCG is it that normal pregnancy can be earliest detected by TVS (transvaginal USG)? \nSelect and explain. \nA. 500 IU/mL, B. 1000 IU/mL, C. 1500 IU/mL, D. 2000 IU/mL', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Ans. is b, i.e. 1000IU/MLRef Dutta Obs. 7/e, p 642Critical titre of hCG:b-hCG level (mIU/ml)Structure visibleTVS/TAS* 1000-1200Gestational sacTVS* 6000Gestational SacTAS', 'role': 'assistant'}]