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
An alternate to lead in the walls of radiology room is: 
Select and explain. 
A. Walls made up of 3 inch concrete, B. Walls made up of barium plaster or barium concrete, C. Walls made up of 3 inch steel, D. All of the above
D: All of the above
Answer: D: All of the above 
Explanation: Walls made of 3" concrete, 3" x 16" of steel or 1 mm of lead, barium plaster or barium concrete will suffice to protect adjacent rooms from radiation.