In the ideal double-slit experiment, when a glass-plate (refractive index 1.5) of thickness t is introduced in the path of one of the interfering beams (wavelength `lambda`), the intensity at the position where the central maximum occurred previously remains unchanged. The minimum thickness of the glass-plate is
A. `2 lambda`
B. `(2lambda)/(3)`
C. `(lambda)/(3)`
D. `lambda`
