This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Magnetic Resonance Imaging, Subject name: Radiology
Contrast used for MRI 
Select and explain. 
A. Gadolinium, B. Radium, C. Iridium, D. TDT
A: Gadolinium
Answer: A: Gadolinium 
Explanation: MRI Contrast Agents : Are contrast agents used to improve the visibility of internal body structures in magnetic resonance imaging(MRI). The most commonly used compounds for contrast enhancement are Gadolinium-based.