The principal role of calcium in skeletal muscle contraction is to

A) participate in the propagation of an action potential along the surface of the muscle fiber.
B) bind troponin, which in turn permits tropomyosin to uncover the cross-bridge binding site on the thin filament.
C) bind tropomyosin, which in turn permits troponin to uncover the cross-bridge binding site on the thin filament.
D) activate a calcium-calmodulin-dependent protein kinase, which in turn phosphorylates and activates myosin.