The degree of dissociation of pure water at $25^{\circ} \mathrm{C}$ is found to be $1.8 \times 10^{-9}$. The dissociation constant, $K_{\mathrm{d}}$ of water, at $25^{\circ} \mathrm{C}$ is
(a) $10^{-14}$
(b) $1.8 \times 10^{-16}$
(c) $5.56 \times 10^{-13}$
(d) $1.8 \times 10^{-14}$