The role of Ca²⁺ in the control of muscle contraction is to

A) cause depolarization of the T tubule system.
B) initiate changes in protein conformation that expose myosin-binding sites on actin.
C) change the conformation of myosin heads,thus causing microfilaments to slide past each other.
D) bind to tropomyosin and break actin-myosin cross-bridges.
E) block the ATP-binding site on myosin heads,enabling muscles to relax.