Physics: Thin- film Interference Problem?

Three experiments involving a thin film (in air) are shown. If t denotes the film thickness and λ denotes the wavelength of the light in the film, which experiments will produce constructive interference as seen by the observer?

A.
I only
B.
II only
C.
III only
D.
I and III only
E.
II and III only
2 answers 2