When an activity is called a critical activity?
A. When earliest start time (est) is equal to the earliest finish time (eft)
B. When earliest start time (est) plus the time taken by it is equal to the latest finish time (lft)
C. When latest start time (lst) plus the time taken by it is equal to the latest finish time(lft)
D. When earliest start time (est) plus the time taken by it is equal to the earliest finish time (eft)