A thin glass plate of thickness is 32500λ ( λ is wavelength of light used) and refractive index μ=1.5 is inserted between one of the slits and the screen in Young's double slit experiment. At a point on the screen equidistant from the slits, the ratio of the intensities before and after the introduction of the glass plate is :
(a) 2:1
(b) 1:4
(c) 4:1
(d) 4:3