Verb narrow down has 2 senses

• narrow down(v = verb.cognition) nail down, narrow, peg down, pin down, specify - define clearly; "I cannot narrow down the rules for this game"
is one way to determine
Sample sentences:
Somebody ----s something
Somebody ----s something PP

• narrow down(v = verb.change) narrow, specialise, specialize - become more focus on an area of activity or field of study; "She specializes in Near Eastern history"
is one way to alter, change, vary
Sample sentences:
Something ----s
Somebody ----s
Something is ----ing PP
Somebody ----s PP