Potassium ion channels open ____, resulting in ____.​

A) as the action potential passes down the axon; a more negatively charged axon cytoplasm​
B) ​when the threshold potential is reached; the opening of more sodium channels
C) ​during the resting potential; the diffusion of sodium ions into the axon cytoplasm
D) ​when the action potential reaches the axon terminal; a more positively charged axon cytoplasm
E) ​when neurotransmitters attach to dendrite receptors; no difference in the charge of the axon cytoplasm