In Young's double slit experiment, the distance between two slits that results in the third minimum for 420 nm violet light at the angle of ${30}^o$, is 

• A

  $8.4\times {10}^{-3}mm$

• B

  $6.8\times {10}^{-3}mm$

• C

  $2.1\times {10}^{-3}mm$

• D

  $4.2\times {10}^{-2}mm$