At 700 K , the equilibrium constant $\mathrm{K}_{\mathrm{p}}$ for the reaction $2 \mathrm{SO}_3(\mathrm{~g}) \rightleftharpoons 2 \mathrm{SO}_2(\mathrm{~g})+\mathrm{O}_2(\mathrm{~g})$ is $1.80 \times 10^{-3}$. What is the numerical value in mole per litre of equilibrium constant $\mathrm{K}_{\mathrm{c}}$ for this reaction at the same temperature?
[2002]
(a) $8.1 \times 10^{-8}$
(b) $9.1 \times 10^{-9} \mathrm{~mol} \mathrm{~L}^{-1}$
(c) $3.1 \times 10^{-7}$
(d) $6.1 \times 10^{-7} \mathrm{~mol} \mathrm{~L}^{-1}$