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
X, Y, Z are three ions permeable through cell membrane X = –50 and Y = –30. If at RMP, when there is no net electrogenic transfer. What is the value of Z? 
Select and explain. 
A. –130, B. 10, C. 80, D. –50
C: 80
Answer: C: 80 
Explanation: At RMP, X + Y + Z = 0
X = –50, Y = –30 , So Z must be +80