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 is: 
Select and explain. 
A. Decremental phenomenon, B. Doesn't obey all or none phenomenon, C. K+ goes from ECF to ICF, D. Threshold stimulus is required
D: Threshold stimulus is required
Answer: D: Threshold stimulus is required 
Explanation: D i.e. Threshold stimulus is required