Which equation is used to work out the efficiency of an energy transfer?
- A$\text{efficiency} = \frac{\text{useful energy output}}{\text{total energy input}}$
- B$\text{efficiency} = \frac{\text{total energy input}}{\text{useful energy output}}$
- C$\text{efficiency} = \frac{\text{total energy output}}{\text{total power input}}$
- D$\text{efficiency} = (\text{useful energy output}) \times (\text{total energy input})$