A runner covers $300\text{ m}$ with an average speed of $3.0\text{ m s}^{-1}$. She then covers a further $300\text{ m}$ at an average speed of $6.0\text{ m s}^{-1}$. What is her average speed for the whole distance of $600\text{ m}$?
- A$2.0\text{ m s}^{-1}$
- B$4.0\text{ m s}^{-1}$
- C$4.5\text{ m s}^{-1}$
- D$8.0\text{ m s}^{-1}$