When troponin binds calcium, A. it changes conformation, twisting tropomyosin and exposing actin-myosin binding sites. B. ion channels open and sodium rushes into the muscle cell. C. it allows tropomyosin to bind actin. D. it changes conformation, exposing the ATP binding site and allowing the actin-myosin bond to break. E. ATP binds to the myosin head, allowing myosin to attach to actin.