Efficiency of a machine is given by—

• a$\dfrac{\text{energy given}}{\text{useful work}} \times 100\%$
• b$\dfrac{\text{useful work}}{\text{energy given}} \times 100\%$
• c$\dfrac{\text{energy wasted}}{\text{energy given}} \times 100\%$
• d$\dfrac{\text{useful work}}{\text{time}} \times 100\%$