The graph illustrates how the velocity of a vehicle travelling in a straight line changes with time $t$. The vehicle is travelling at $4.0\,\text{m s}^{-1}$ and starts to accelerate at $t = 0$. What is the acceleration of the vehicle at $t = 3.0\,\text{s}$?
- A$0.67\,\text{m s}^{-2}$
- B$1.0\,\text{m s}^{-2}$
- C$1.3\,\text{m s}^{-2}$
- D$2.0\,\text{m s}^{-2}$