In an 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

  $\text{2λ}$

• B

  $2\mathrm{\lambda }/3$

• C

  $\mathrm{\lambda }/3$

• D

  $\lambda$