This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: G.I.T, Subject name: Anatomy
[{'content': 'The substance used in OCG is \nSelect and explain. \nA. Iopanoic acid, B. Sodium diatrozite, C. Meglumine iodothalamate, D. Biligraffin', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Iopanoic acid is used in oral cholecystography Biligraffin is used in IV cholangiography Ref: Sabiston 20th edition Pgno :1487', 'role': 'assistant'}]