Myocardial cells exhibit a plateau phase instead of quickly repolarizing. This is due to ________.
1) inward diffusion of calcium through slow Ca²⁺ channels.
2) inward diffusion of sodium through fast Na⁺ channels.
3) inward diffusion of potassium through voltage-gated K⁺ channels.
4) outward pumping of sodium by the Na⁺/K⁺ pump.