What Explains Why There Is a Threshold Depolarization of About

What explains why there is a threshold depolarization of about 45 mV above resting membrane potential before any transmitter release occurs?

A) This is the magnitude of depolarization required to activate presynaptic voltage gated sodium channels.
B) This is the magnitude of depolarization required to activate presynaptic voltage gated potassium channels.
C) This is the magnitude of depolarization required to activate a presynaptic action potential.
D) This is the magnitude of depolarization required to activate presynaptic voltage-gated calcium channels.
E) This is the magnitude of depolarization required to activate a postsynaptic action potential.