Task: Question Answering
Input (English): 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?  
 (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.
Output (English): B