The figure shows a Young's double slit experimental setup. It is observed that when a thin transparent sheet of thickness t and refractive index is put in front of one of the slits, the central maximum gets shifted by a distance equal to n fringe widths. If the wavelength of light used is λ, t will be:

• A. \(\frac{{2{\rm{nD\lambda }}}}{{{\rm{a}}\left( {{\rm{\mu }} - 1} \right)}}\)
• B. \(\frac{{{\rm{nD\lambda }}}}{{{\rm{a}}\left( {{\rm{\mu }} - 1} \right)}}\)
• C. \(\frac{{{\rm{D\lambda }}}}{{{\rm{a}}\left( {{\rm{\mu }} - 1} \right)}}\)
• D. \(\frac{{2{\rm{D\lambda }}}}{{{\rm{a}}\left( {{\rm{\mu }} - 1} \right)}}\)