This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Contrast Agents, Subject name: Radiology
Which of the following is Iso-osmolar agent? 
Select and explain. 
A. Ionic monomer, B. Ionic dimer, C. Non-ionic monomer, D. Non-ionic dimer
D: Non-ionic dimer
Answer: D: Non-ionic dimer 
Explanation: - Ionic Monomer - High osmolality contrast media - Ionic Dimer - Low osmolality contrast media - Non-ionic Monomer - Low osmolality contrast media - Non-ionic Dimer - Iso osmolality contrast media