Which of the following properties of a neuron's action potential determines the maximum firing rate (e. g. determines the number of action potentials that can be fired per second)? a. The duration that it takes for the neuron to integrate all the EPSP and IPSPs and "decide" if threshold has been met. b. The duration it takes for all voltage-gated K+ channels to close during the afterpotential and the Na+/K+ pump to redistribute these ions so as to restore the resting potential. c. The duration that it takes for neurotransmitter to diffuse across a synapse. d. The duration that it takes for all voltage-gated Na+ channels to return to a closed, but active (not inactivated) state after the initial rapid depolarization "spike".