In an ideal Young's double-slit experiment,a glass plate of thickness $t$ and refractive index $\mu = 1.5$ is placed in the path of one of the interfering beams. If the central bright fringe shifts to the position originally occupied by the first bright fringe (corresponding to wavelength $\lambda$),then the minimum thickness $t$ of the glass plate is:

• A
  $2\lambda$
• B
  $\frac{2\lambda}{3}$
• C
  $\frac{\lambda}{3}$
• D
  $\lambda$