Refer to the diagram of a sarcomere below.Which statement based on the diagram is true?

A) The structure 1 consists of two chains made of monomers.
B) The structure 1 shortens in length as contraction occurs.
C) The structure 2 has a head that hydrolyzes ATP.
D) The zone labeled 3 shortens in length as contraction occurs.
E) The unit labeled 4 maintains a constant length as contraction occurs.