During the Plateau Phase of the Action Potentials of Myocardial

During the plateau phase of the action potentials of myocardial contractile cells,which ion(s) is/are crossing the membrane?

A) Ca²⁺ only
B) K⁺ only
C) Na⁺ only
D) both Ca²⁺ and K⁺
E) both Na⁺ and K⁺