Sarcoplasmic reticulum is a specialized endoplasmic reticulum known for its role in muscle fiber contraction.
Which of the following statements accurately defines sarcoplasmic reticulum?
A. a modified smooth endoplasmic reticulum that wraps around the myofibrils and stores calcium ions.
B. Dome-shaped muscle located between the thoracic and abdominopelvic cavities.
C. Flat muscle over the anterior, superficial abdomen.
D. An extension of the sarcolemma that dives into the muscle fiber and surrounds myofibrils.