A particle is moving such that its position vector varies with time as $\overrightarrow{\mathrm{r}}=\left(1-\mathrm{\alpha t}\right)\mathrm{t}\overrightarrow{\mathrm{A}}$ where $\mathrm{\alpha } \mathrm{and} \overrightarrow{\mathrm{A}}$ are constant quantities. At t = 0, the particle is at a position O. At some later instant 't₀' , the particle is again at O. Velocity of the particle at the instant t₀ is: