A slit of width a is illuminated by white light. The first minimum for red light 
(  $\lambda$= 6500 Å) will fall at  $\mathrm{\theta }$ = 30° when a will be

• A

  $3250 \mathrm{\AA }$

• B

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

• C

  1.3 micron    

• D

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