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: Ophthalmology
In the laser machines used in ophthalmology, the atomic environment used consists of: 
Select and explain. 
A. Crystal rod, B. Fluid-filled cavity, C. Gas-filled cavity, D. Any of the above
D: Any of the above
Answer: D: Any of the above 
Explanation: Ans. Any of the above