The speed-time graph below represents a car travelling in a straight line. What is the acceleration of the car at time $40\,\text{s}$?
- A$0\,\text{m s}^{-2}$
- B$\frac{15 - 3}{40}\,\text{m s}^{-2}$
- C$\frac{15}{40}\,\text{m s}^{-2}$
- D$(15 - 3)\,\text{m s}^{-2}$