Which Statement Describes Slow Depolarization

Which statement describes slow depolarization?

A) Allows dehydrated sodium ions to enter the negatively charged axoplasm causing the threshold for firing threshold
B) Allows release of the sodium ions from the receptor sites in the ion channels allowing negative ions to enter the axoplasm
C) Occurs until the axoplasm has depolarized 15 to 20 mV to reach -50 to -55 mV, which is the firing threshold
D) Occurs until the calcium ions rebind with the receptor sites to reach firing threshold at -50 to -55 mV