Muscle relaxation occurs when:
a. calcium ions are actively transported out of the sarcoplasmic reticulum
b. when the motor neuron continues releasing Acetylcholine into the neuromuscular junction
c. calcium ions are actively transported into the sarcoplasmic reticulum
d. calcium ions diffuse out of the sarcoplasmic reticulum
e. calcium ions diffuse into the sarcoplasmic reticulum