What Is the Stoichiometry of the NCKX Transport System

What is the stoichiometry of the NCKX transport system?

A) One calcium and one potassium ion outward for every four sodium ions entering the cell (4 Na⁺:1 Ca²⁺:1K⁺)
B) Three potassium ions outward for every three sodium ions and one calcium entering the cell (3 Na⁺:1 Ca²⁺:3K⁺)
C) Two calcium ions outward for every two protons and one potassium entering the cell (2 H⁺:2 Ca²⁺:1K⁺)
D) One ion X outward for every one sodium, one potassium and one calcium ions entering the cell (1Na⁺:1 Ca²⁺:1K⁺:1 X)
E) Na⁺, K⁺ and Ca²⁺ are not transported in a fixed ratio through the NCX transport system