This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General obstetrics, Subject name: Gynaecology & Obstetrics
[{'content': 'Low-lying placenta is when the edge of placenta is __________ \nSelect and explain. \nA. <2cm away from the internal os, B. >2.5 cm away from the internal os, C. >2 cm away from the internal os, D. <2.5 cm away from the internal os', 'role': 'user'}, {'content': 'Answer: A \nExplanation: When the placental edge is <2cm from, but not over, the internal os, the placenta is defined as low-lying placenta. The diagnosis of low-lying placenta is made by the ultra sound examination. Ref: Williams Obstetrics 25th edition Pgno: 774', 'role': 'assistant'}]