The rate constant of reaction $2\mathrm{A}+\mathrm{B}\to \mathrm{C}\hspace{0.17em}\mathrm{is} 2.57\times {10}^{-5} \mathrm{L} {\mathrm{mole}}^{-1}{\sec }^{-1}$ after 10 sec, $2.65\times {10}^{-5} \mathrm{L} {\mathrm{mole}}^{-1}{\sec }^{-1}$ after 20 sec and $2.55\times {10}^{-5} \mathrm{L} {\mathrm{mole}}^{-1}{\sec }^{-1}$ after 30 sec. The order of the reaction is: