A radio-controlled toy car moves in a straight line for $15\,\text{s}$. The graph shows how the velocity $v$ of the car changes with time $t$. What is the average velocity of the toy car for the journey represented by the graph?
- A$-1.5\,\text{m s}^{-1}$
- B$0.0\,\text{m s}^{-1}$
- C$4.0\,\text{m s}^{-1}$
- D$4.5\,\text{m s}^{-1}$