An athlete covers 300 metres uphill at a constant speed of $3.0\,\text{m s}^{-1}$. She then straight away covers the same distance downhill at a constant speed of $6.0\,\text{m s}^{-1}$. What is her average speed for the 600 metre journey?
- A$2.0\,\text{m s}^{-1}$
- B$3.0\,\text{m s}^{-1}$
- C$4.0\,\text{m s}^{-1}$
- D$4.5\,\text{m s}^{-1}$