In double slit experiment fringes are obtained using light of wavelength `4800 Å` One slit is covered with a thin glass film of refractive index. `1.4` and another slit is covered by a film of same thickness but refractive index `1.7`. By doing so, the central fringe is shifted to fifth bright fringe in the original pattern. The thickness of glass film is
A. `2xx10^(-3)mm`
B. `4xx10^(-3)mm`
C. `6xx10^(-3)mm`
D. `8xx10^(-3)mm`
