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: Physiology
[{'content': 'Normal prothrombin time is: \nSelect and explain. \nA. 5-7 sec, B. 10-20 sec, C. 30 sec, D. 1 min', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]