In a distance vector algonthm, when can a node update its distance$-$vector estimate?

a$.$ When it "sees' a cost change in one of its directly attached links

b$.$ When it receives a distance vector update from some neighbor

c$.$ When a neighbor "sees" a cost change in one of its directly attached links

d$.$ coly a and $b$

a$.$ only a and $c$

anly band $c$

none of the atove