Which of the Following Statements Describes Phase 4 of the Action

Which of the following statements describes phase 4 of the action potential of cells in the sinoatrial (SA) node?

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