The Outside of a Neuron Is Positively Charged at Resting

The outside of a neuron is positively charged at resting potential. Why do K⁺ ions leave the cell when K⁺ channels open during an action potential?

A) The diffusion gradient is the only factor that affects ion movement.
B) K⁺ ions are actively pumped out by the sodium-potassium pump acting in reverse.
C) Voltage-gated K⁺ channels open when membrane potential reaches +50 mV during the depolarization phase. The diffusion gradient and electrical force cooperate to drive K⁺ ions out of the cell.
D) Negatively charged proteins leave the cell at the onset of the action potential.