Activity D on a CPM network has predecessors B and C$,$ and has successor F$.$ D has duration $6.$ B$\text{'}$s earliest finish is $18$ while $C\text{'}$s is $20.$ F$\text{'}$s late start is $26.$ Which of the following is definitely true?

Select one:

a$.$ Dis critical, and has zero slack.

b$.$ is completed before $B.$

C$.$ D has no slack but is not critical.

d$.F$ is critical and has zero slack.

e$.B$ is a critical activity.