A slit of width a is illuminated by white light. For red light ($\mathrm{\lambda }$ = 6500 Å), the first minima is obtained at $\mathrm{\theta }=30°$. Then the value of a will be   

• A

  $3250\text{Å}$

• B

  $6.5\times {10}^{-4}\mathrm{mm}$

• C

  $1.24\mathrm{microns}$

• D

  $2.6\times {10}^{-4}\mathrm{cm}$