During the contraction of a vertebrate skeletal muscle fiber, calcium ions

A) break cross-bridges by acting as a cofactor in the hydrolysis of ATP.
B) bind with troponin, changing its shape so that the myosin-binding sites on actin are exposed.
C) transmit action potentials from the motor neuron to the muscle fiber.
D) spread action potentials through the T tubules.
E) re-establish the polarization of the plasma membrane following an action potential.