$A$ Fraunhofer diffraction pattern due to a narrow slit is obtained on a screen placed at a distance $D$ from the slit whose slit width is $a$. The distance of the first secondary maximum from the central maximum is

• A
  $\frac{3 D \lambda}{a}$
• B
  $\frac{3 D \lambda}{2 a}$
• C
  $\frac{2 D \lambda}{3 a}$
• D
  $\frac{2 D \lambda}{a}$