The Absolute Refractory Period Occurs Because

Multiple Choice

The absolute refractory period occurs because 

A) the cell is hyperpolarized, requiring a larger than normal stimulus in order to form another action potential. 
B)  it takes time for the sodium-potassium pumps to return ion concentrations to the normal resting state.​ 
C)  only one action potential at a time can travel the length of an axon.​ 
D)  sodium channels cannot reopen until the cell regains its resting potential.​