Light of wavelength $\lambda$ is incident on a slit of width $a$. The resulting diffraction pattern is observed on a screen placed at a distance $D$. If the linear width of the central maximum is equal to the width of the slit,then $D = $ . . . . . . .

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