-What Is the Structure Indicated by the Number 4 in Figure

-What is the structure indicated by the number 4 in Figure 12.1 and what is its functional significance?

A) sarcomere : the functional unit of muscle contraction whose length is changed as a muscle shortens and lengthens
B) H zone : area of myosin not overlapped by actin
C) A band : length of myosin attached to an M line
D) Z line : anchors the thin filaments together
E) M line : where the tails of the myosin molecules are bound to one another