Which of the following is used to calculate free float of an activity？（ ）

A、EarliestFinishTime(EF)-LatestFinishTime(LF)
B、EarliestFinishTime(EF)-EarliestStartTime(ES)
C、LatestFinishTime(LF)-LatestStartTime(LS)
D、LatestStartTime(LS)-EarliestStartTime(ES)