Question

lo 4.4. what structure in a muscle causes it to shorten?
myofibril
sarcomere
fascicle
tendon
z - line

Explanation:

Sarcomeres are the basic contractile units of muscle fibers. Muscle - shortening occurs due to the sliding of actin and myosin filaments within sarcomeres. Myofibrils are made up of sarcomeres, fascicles are bundles of muscle fibers, tendons attach muscle to bone, and Z - lines are boundaries of sarcomeres.

Answer:

B. Sarcomere