For a chemical reaction $A \rightarrow B$, the rate of the reaction is $2 \times 10^{-3}$ mol dm$^{-3}$ s$^{-1}$ when the initial concentration is $0.05$ mol dm$^{-3}$. The rate of the same reaction is $1.6 \times 10^{-2}$ mol dm$^{-3}$ s$^{-1}$ when the initial concentration is $0.1$ mol dm$^{-3}$. What is the order of the reaction?