Work Breakdown Structures typically include one or more intermediate levels. Which of the following statements correctly describe the nature of these intermediate levels?

a$.$ They generally represent items that need to be created in order to produce the final deliverables.

b$.$ They generally represent the raw materials required to create the final deliverables.

c$.$ They generally represent the resource assignments to create the final deliverables.

d$.$ They generally represent the sequence and timing of work packages