For the decomposition of a compound AB at 600 K, the following data were obtained:

| [AB] mol dm$^{-3}$ | Rate of decomposition of AB in mol dm$^{-3}$ s$^{-1}$ |
|---|---|
| 0.20 | $2.75 \times 10^{-8}$ |
| 0.40 | $11.0 \times 10^{-8}$ |
| 0.60 | $24.75 \times 10^{-8}$ |

The order for the decomposition of AB is