Red Light Is Viewed Through a Thin Vertical Soap Film $\lambda$

Multiple Choice

Red light is viewed through a thin vertical soap film. At the third dark area shown, the thickness of the film, in terms of the wavelength within the film, is:

A) $\lambda$
B) 3 $\lambda$ /4
C) $\lambda$ /2
D) $\lambda$ /4
E) 5 $\lambda$ /4