Question: 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?

  Options: 
 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. 
Answer: B