Here's a problem to solve: A perpetual motion machine is a theoretical device which, once started, continues to operate without any additional input of energy. Which statement describes why a perpetual motion machine is impossible to design?

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

- A: Energy can be converted into mass.
 
- B: Friction reduces the efficiency in a system.
 
- C: The amount of energy in a system remains constant.
 
- D: Potential energy can be converted into kinetic energy.
 