A Puddle of Water Has a Thin Film of Gasoline

Multiple Choice

A puddle of water has a thin film of gasoline floating on it.A beam of light is shining perpendicular on the film.If the wavelength of light incident on the film is 560 nm and the indices of refraction of gasoline and water are 1.40 and 1.33,respectively,what must be the minimum thickness of the film to see a bright reflection?

A) 100 nm
B) 200 nm
C) 300 nm
D) 400 nm
E) 500 nm