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: Physiology
Action potential across the cell membrane is maintained by? 
Select and explain. 
A. Na + K+  pump, B. cAMP, C. Ca ++, D. Phosphodiesterase
A: Na + K+  pump
Answer: A: Na + K+  pump 
Explanation: None