During the Contraction of a Vertebrate Skeletal Muscle Fiber,calcium Ions

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.