Light from a HeNe Laser (632 $\Delta$ LDestructive Interference Is Observed in the Initial Setup

Light from a HeNe laser (632.8 nm) is incident upon an interferometer in which the two paths for light to travel differ by an amount $\Delta$ L.Destructive interference is observed in the initial setup.Then,one of the paths is then shortened,and as this occurs,the interference goes from destructive slowly to constructive to destructive,and finally to constructive again.By what amount was the path length shortened?

A) 950 nm
B) 1900 nm
C) 1270 nm
D) 320 nm