The Role of Ca²⁺ in the Control of Muscle Contraction

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

A) cause depolarization of the T tubule system.
B) change the conformation of troponin, thus exposing myosin-binding sites.
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.