The project to provide an organizational restructuring of the company to increase operational efficiency and provide clearer lines of communication and development opportunities for managers has been initiated. During the planning processes, the project manager is determining the milestones. Of the following, what is a key element of a milestone definition?

a. It has a duration of zero (0)

b. It defines a phase of the project

c. It has a duration of one day

d. It has value in the project charter but not in the plan