A flake of glass (refractive index 1.5) is placed over one of the openings of a double slit apparatus. The interference pattern displaces itself through seven successive maxima towards the side where the flake is placed. if wavelength of the diffracted light is $\mathrm{\lambda }=600 \mathrm{nm}$, then the thickness of the flake is

• A

  2100 nm

• B

  4200 nm

• C

  8400 nm

• D

  None of these