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 i,,) the intensity at the position where the central maximum occurred previously remains unchanged. The minimum thickness of the glass-plate is

• A

  $2\mathrm{\lambda }$

• B

  $2\mathrm{\lambda }/3$

• C

  $\mathrm{\lambda }/3$

• D

  $\lambda$