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
[{'content': 'Magnitude of action potential is mainly affected by: September 2005 \nSelect and explain. \nA. Calcium ion, B. Hydrogen ion, C. Sodium ion, D. Potassium ion', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Ans. C: Sodium ion', 'role': 'assistant'}]