What is an accurate difference between the Program Evaluation and Review Technique $($PERT$)$ and critical path method $($CPM$)?$

a$.$ CPM involves more work than PERT because it requires several duration estimates.

b$.$ CPM addresses the risk associated with duration estimates whereas PERT does not.

c$.$ Unlike CPM$,$ PERT estimates only when there is no risk of uncertainty.

d$.$ PERT uses different duration estimates whereas CPM uses one specific duration estimate.