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: Medicine
[{'content': 'Azotemia occurs when - \nSelect and explain. \nA. GFR 50% of normal, B. GFR 80% of normal, C. GFR 20% - 50% of normal, D. GFR 25% of normal', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]