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': 'The substance used of OCG is: \nSelect and explain. \nA. lapanoic acid, B. Sodium diatrozite, C. Melglumine iodothalamate, D. Biligraffin', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. lapanoic acid', 'role': 'assistant'}]