What Is the Purpose of the Constant Field Equation

What is the purpose of the constant field equation?

A) To determine the equilibrium potential of a single ion
B) To determine the membrane potential when potassium and chloride are present.
C) To determine the resting potential of a cell given all ionic concentrations and conductances
D) To determine which ions are moving across the membrane at rest
E) To determine the minimum voltage at which ionic movement can be balanced