This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Fundamentals in Radiology, Subject name: Radiology
Which of the following statements about contrast in radiography is true - 
Select and explain. 
A. Ionic monomers have three iodine atoms per two paicles in solution, B. High osmolar contrast agents may be ionic or non ionic, C. Gadolinum may cross the blood brain barrier, D. Iohexol is a high osmolar contrast media
A: Ionic monomers have three iodine atoms per two paicles in solution
Answer: A: Ionic monomers have three iodine atoms per two paicles in solution 
Explanation: option A is correct : Ionic monomers have three iodine atoms per two paicles in solution Ionic dimers and non ionic monomers have three iodine atoms per one paicle in solution Non ionic dimers have six iodine atoms per one paicle in solution option B is incorrect : High osmolar agents (ionic monomers) are only ionic low osmolar agents may be ionic or non ionic iso osmolar agents are only non ionic option C is incorrect : Gadolinium does not cross Blood brain barrier (BBB) option D is incorrect : Iohexol is low osmolar (non ionic monomer)