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: Radiology
[{'content': "'Rim' and 'ball' nephrograms in intravenous urography are seen in – \nSelect and explain. \nA. Normal kidneys, B. Acute obstructive nephropathy, C. Chronic obstructive nephropathy, D. Chronic renal failure", 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]