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
The substance used of OCG is: 
Select and explain. 
A. lapanoic acid, B. Sodium diatrozite, C. Melglumine iodothalamate, D. Biligraffin
A: lapanoic acid
Answer: A: lapanoic acid 
Explanation: Ans. lapanoic acid