A runner covers $300\ \text{m}$ at 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 over 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$8.0\ \text{m s}^{-1}$