The pH of a $$1.0 imes10^{-3}\ \mathrm{mol\,L^{-1}}$$ solution of a weak acid $$\mathrm{HA}$$ is $$3.60$$. The dissociation constant of the acid is

A

$$8.4 \times10^{-8}$$

B

$$8.4 \times10^{-6}$$

C

$$8.4 \times10^{-5}$$

D

$$8.4 \times10^{-2}$$