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': 'Contrasts used in USG – \nSelect and explain. \nA. Urograffin, B. Ultragraffin, C. Sonavist, D. Conray', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]