In a biprism experiment,a wavelength $\lambda$ is used to obtain an interference pattern. The fringe width is $W_1$ at a distance $D_1$ from the screen. When the screen is moved towards the biprism,the fringe width becomes $W_2$ at a distance $D_2$. The distance between the two virtual images of the slit is:

• A
  $\frac{\lambda(D_2-D_1)}{(W_1-W_2)}$
• B
  $\frac{\lambda(W_1-W_2)}{(D_1-D_2)}$
• C
  $\frac{\lambda(W_2-W_1)}{(D_1-D_2)}$
• D
  $\frac{\lambda(D_1-D_2)}{(W_1-W_2)}$