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: Surgery
What is the purpose of Nitrogen gas filled in anaesthetic cartridge: 
Select and explain. 
A. To maintain the pressure in the cartridge., B. To prevent deterioration of vasoconstrictor., C. To prevent deterioration of LA., D. All of the above.
B: To prevent deterioration of vasoconstrictor.
Answer: B: To prevent deterioration of vasoconstrictor. 
Explanation: The purpose of nitrogen gas-filled bubbles in the anaesthetic cartridge is to prevent deterioration of the vasoconstrictor.