During the Contraction of a Vertebrate Skeletal Muscle Fibre, Calcium

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

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