A man of mass $80\,\text{kg}$ attaches himself to one end of a rope that passes over a single fixed pulley. He pulls the free end of the rope so that he rises with an average speed of $50\,\text{cm s}^{-1}$. What is the average useful power at which he is working?
- A$40\,\text{W}$
- B$0.39\,\text{kW}$
- C$4.0\,\text{kW}$
- D$39\,\text{kW}$