In a biprism experiment, the fifth dark fringe is obtained at a point. $A$ thin transparent film of refractive index '$\mu$' is placed in one of the interfering paths. Now, the $7^{th}$ bright fringe is obtained at the same point. If '$\lambda$' is the wavelength of light used, the thickness of the film is equal to:

• A
  $\frac{1.5(\mu - 1)\lambda}{(\mu - 1)}$
• B
  $\frac{1.5 \lambda}{(\mu - 1)}$
• C
  $\frac{2.5(\mu - 1)\lambda}{(\mu - 1)}$
• D
  $\frac{2.5 \lambda}{(\mu - 1)}$