A girl whose mass is $50\,\text{kg}$ runs up a staircase of $20$ steps in $7.0\,\text{s}$. Each step has a height of $25\,\text{cm}$. What is the useful average power output provided by the girl in climbing the staircase?
- A$18\,\text{W}$
- B$36\,\text{W}$
- C$350\,\text{W}$
- D$2500\,\text{W}$