During an action potential, the phase where ___________ moves into the cell results in depolarization while the phase where ______________ exits the cell results in repolarization. 

• A. Na$^+$: K$^+$
• B. Ca$^2$$^+$: Na$^+$
• C. Na$^+$: Ca$^2$$^+$
• D. K$^+$: Ca$^2$$^+$