During which period is a muscle fiber unable to respond to a rapid succession of threshold stimuli, and why does this happen?
a) Refractory period; Due to the temporary inactivation of sodium channels
b) Depolarization phase; Due to excessive calcium influx
c) Recovery period; Due to lactic acid buildup
d) Contractile phase; Due to ATP depletion
e) Excitation phase; Due to potassium efflux