Light of wave-length $\lambda$ is incident on a slit of width $\mathrm{d}$. The resulting diffraction pattern is observed on a screen placed at a distance $\mathrm{D}$. The linear width of the principal maximum is equal to the width of the slit, then $\mathrm{D}=$
(A) $\left(\mathrm{d}^{2} / 2 \lambda\right)$
(B) $\left(2 \lambda^{2} / \mathrm{d}\right)$
(C) $(\mathrm{d} / \lambda)$
(D) $(2 \lambda / \mathrm{d})$