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': 'Best modality to diagnose unruptured tubal pregnancy is : \nSelect and explain. \nA. Vaginal ultrasound, B. Serum hCG estimation, C. Serum Progesterone estimation, D. Culdocentesis', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Vaginal ultrasound', 'role': 'assistant'}]