During the contraction of a vertebrate skeletal muscle fibre,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 fibre.
D)spread action potentials through the T tubules.
E)re-establish the polarization of the plasma membrane following an action potential.