Autorhythmicity is a unique feature of cardiac pacemaker cells. What feature of cardiac pacemaker cells allows them to be autorhythmic while other cardiac cells are not?

• A. Pacemaker cells use Ca$^+$$^2$ ions, unlike other action potentials.
• B. Sodium channels in pacemaker cells allow both calcium and sodium to pass through.
• C. The pacemaker potential is stimulated by polarization of the cell.
• D. Unlike other action potentials, the first channel to open is the potassium channel, leading to repolarization.