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: Pharmacology
Dissociative sedation and analgesia described by Robert and Rosebaum in 1971 is: 
Select and explain. 
A. 5 to 20% N2O, B. 20 to 30% N2O, C. 30 to 50% N2O, D. 50 to 70% N2O
D: 50 to 70% N2O
Answer: D: 50 to 70% N2O 
Explanation: None