Which statement correctly describes what causes the second voltage-gated na+ channel to open? a) as na+ ions enter the cell through the first channel, they spread out from the channel. when these na+ ions reach the second channel, it opens. b) as na+ ions enter the cell through the first channel, na+ ions outside the cell move toward the open na+ channel. when the concentration of na+ ions near the second channel becomes low enough, the second channel opens. c) after the first channel opens, the movement of na+ ions (both inside and outside the cell) alters the na+ ion distribution across the membrane near the second channel, causing it to open. d) after the first channel opens, the movement of many types of ions (both inside and outside the cell) alters the distribution of charges near the second channel, causing it to open.