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: Anaesthesia
If pneumocephalus created either by surgery or by performance of a pneumoencephalogram, it is suggested that nitrous oxide be avoided for how many days – 
Select and explain. 
A. 4 days, B. 5 days, C. 6 days, D. 7 days
D: 7 days
Answer: D: 7 days 
Explanation: Once the pneumoencephalum has developed nitrous oxide should not be used for 1 week.