In Young's double slit experiment, the distance between two slits that results in the third minimum for 680 nm red light at the angle of ${30}^o$, is 

• A

  $3.4\times {10}^{-3} \mathrm{mm}$

• B

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

• C

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

• D

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