During the Contraction of a Vertebrate Skeletal Muscle Fibre, Calcium

During the contraction of a vertebrate skeletal muscle fibre, 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 fibre.
D) spread action potentials through the T tubules.