$A$ light wave of wavelength $\lambda$ is incident on a slit of width $d$. The resulting diffraction pattern is observed on a screen at a distance $D$. If the linear width of the principal maximum is equal to the width of the slit,then the distance $D$ is:

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