Lawrie Ryan and Roger Norris Solutions for Chapter: Reaction Kinetics, Exercise 14: Question

The reaction $2\mathrm{NO}\left(\mathrm{g}\right)+{\mathrm{Cl}}_2\left(\mathrm{g}\right)\to 2\mathrm{NOCl}\left(\mathrm{g}\right)$ is carried out in a sealed tube. At the start of the experiment, the concentration of both $\mathrm{NO}$ and $\mathrm{Cl}$, was $0.01 \mathrm{mol} {\mathrm{dm}}^{-3}$ The reaction is second order with respect to $\mathrm{NO}$ and first order with respect to $\mathrm{Cl}$. Which one of these represents the rate of reaction when the concentration of $\mathrm{NO}$ falls to $0.005 \mathrm{mol} {\mathrm{dm}}^{-3}$?