A thin oil film (index of refraction n=1.40) floats on water (n=1.33). Monochromatic light of wavelength λ in air is incident normally from above. For the reflected light to exhibit constructive interference, which thickness condition must the oil satisfy?
a) 2nt=(m+41)λ
b) 2nt=mλ
c) 2nt=(m+21)λ
d) nt=(m+21)λ