During the Contraction of a Vertebrate Skeletal Muscle Fiber, Calcium

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

A) break cross-bridges as a cofactor in 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