If the width of the slit is $a$,then the value of the first secondary maximum in a single slit diffraction pattern is given by:

• A
  $a \sin \theta = \frac{\lambda}{2}$
• B
  $a \cos \theta = \frac{3\lambda}{2}$
• C
  $a \sin \theta = \lambda$
• D
  $a \sin \theta = \frac{3\lambda}{2}$