In a Young's double slit experiment,the fringe width will remain the same if ($D =$ distance between screen and plane of slits,$d =$ separation between two slits,and $\lambda =$ wavelength of light used).

• A
  Both $\lambda$ and $D$ are doubled.
• B
  Both $d$ and $D$ are doubled.
• C
  $D$ is doubled but $d$ is halved.
• D
  $\lambda$ is doubled but $d$ is halved.