$r=k[C O]^{2}$

Let initial concentration of $CO$ is a i.e. $[C O]=a$

$r_{1}=k(a)^{2}=k a^{2}$

when concentration becomes doubled, i.e. $[\mathrm{CO}]=2 a$

$\therefore r_{2}=k(2 a)^{2}=4\, k a^{2}$

$\therefore \quad r_{2}=4 r_{1}$

So, the rate of reaction becomes $4\,times$ .