Rate law for a reaction between $A$ and $B$ is given by $R=k[A]^{n}[B]^{m}$. If concentration of $A$ is doubled and concentration of $B$ is halved from their initial value,the ratio of new rate of reaction to the initial rate of reaction $\left(\frac{r_2}{r_1}\right)$ is

• A
  $2^{(n-m)}$
• B
  $(n-m)$
• C
  $(m+n)$
• D
  $\frac{1}{2^{m+n}}$