Four different children each run up the same staircase. Which child has the greatest useful power when climbing the stairs?
- Amass $40\,\text{kg}$, time taken $15\,\text{s}$
- Bmass $50\,\text{kg}$, time taken $25\,\text{s}$
- Cmass $60\,\text{kg}$, time taken $25\,\text{s}$
- Dmass $70\,\text{kg}$, time taken $15\,\text{s}$