Light of Wavelength 640 Nm Is Incident Perpendicularly from Air

Multiple Choice

Light of wavelength 640 nm is incident perpendicularly from air onto a film 1000 nm thick and of 1.60 refractive index.When part of the light enters the film and is reflected back at the second face,the number of wavelengths contained along the path of this light in the film is

A) 1.0
B) 1.6
C) 2.0
D) 3.2
E) 5.0