Here's a problem to solve: Why is the output energy of a machine always less than the input energy?

Among the 4 following options, which is the correct answer?

- A: Some of the energy is destroyed as the machine operates.
 
- B: The machine must overcome the constant force of magnetism.
 
- C: Useful energy decreases as energy is transferred to the surroundings.
 
- D: Some of the energy is released as heat when converted.
 