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
The substance used in OCG is 
Select and explain. 
A. Iopanoic acid, B. Sodium diatrozite, C. Meglumine iodothalamate, D. Biligraffin
A: Iopanoic acid
Answer: A: Iopanoic acid 
Explanation: Iopanoic acid is used in oral cholecystography Biligraffin is used in IV cholangiography Ref: Sabiston 20th edition Pgno :1487