Which of the following statements describes phase 4 of the action potential of cells in the sinoatrial (SA) node?
A. Potassium permeability is allowing the cell membrane to remain depolarized, and Ca2+ channel opens moving Ca2+ back into the cell.
B. The cells are capable of responding to a greater than normal stimulus before the resting membrane potential is reached.
C. A slow depolarization occurs when Na+ is transported out of the cell and K+ moves back in, resulting in resting membrane potential.
D. The fast sodium channels in the cellular membranes close, causing an abrupt decrease in intracellular positivity.