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': 'Cervical dystocia is usually present at : \nSelect and explain. \nA. Level of external os, B. Level of internal os, C. Level of isthmus, D. Level of cervical canal', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Level of external os', 'role': 'assistant'}]