Analogy type questions:
$(1)$ The portion between two successive $Z$-lines is the functional unit of contraction,which is called the sarcoplasmic reticulum / sarcomere.
$(2)$ The thick filaments in the $A$-band are held together in the middle of this band by a thin fibrous membrane. It is called the $Z$-line / $M$-line.

• A
  $(1)$ Sarcomere,$(2)$ $M$-line
• B
  $(1)$ Sarcoplasmic reticulum,$(2)$ $Z$-line
• C
  $(1)$ Sarcomere,$(2)$ $Z$-line
• D
  $(1)$ Sarcoplasmic reticulum,$(2)$ $M$-line