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
Which of the following is least permeable in an excitable tissue at rest? 
Select and explain. 
A. Na+, B. K+, C. Cl-, D. Ca2+
A: Na+
Answer: A: Na+ 
Explanation: The ratio of permability between K : Cl : Na is 1: 0.45 : 0.04.