When the Cell Membrane of a Vertebrate Skeletal Muscle Is

When the cell membrane of a vertebrate skeletal muscle is depolarized, ryanodine receptors change configuration and permit passage of Ca²⁺

A) passively, from the extracellular fluid to the cytoplasm.
B) passively, from the sarcoplasmic reticulum to the cytoplasm.
C) actively, from the extracellular fluid to the cytoplasm.
D) actively, from the sarcoplasmic reticulum to the cytoplasm.