Which statement best describes a cause and its effect that occurs during the process of muscle contraction? a. the presence of calcium ions within nerve cells causes the sarcomere to shorten. b. the release of calcium ions causes myosin and actin to attach to each other. c. the shortening of the sarcomere causes the release of calcium around the myofibrils. d. the pull of myosin on actin causes the motor neuron to send a nerve impulse.

The correct answer is option B, that is, the release of calcium ions makes actin and myosin to combine with each other.  

At the time of the stimulation for a muscle contraction, calcium is discharged into the fiber. It combines with troponin and instigates it to move tropomyosin, exposing the binding site for the myosin cross-bridge. Thus, the face of the actin filament is now free to combine with myosin heads to initiate contraction of the muscle.  

not sure what it is