Movement of chloride ions across the cell membrane from the the outside of nerve cells to the inside has a delta-G of about 1.1 kcal/mol even in the presence of a gradient. Why is the gradient alone not enough to get the ions into the cell? (Think about how delta-G is calculated for movement into a cell.)

a) must also take into consideration other ion concentrations
b) must also take into consideration the membrane potential
c) must also take into consideration the temperature
d) must also take into consideration cotransport