You will be given a definition of a task first, then some input of the task.
You are given a science question (hard-level) and four answer options (associated with "A", "B", "C", "D"). Your task is to find the correct answer based on scientific facts, knowledge, and reasoning. Do not generate anything else apart from one of the following characters: 'A', 'B, 'C', 'D'. There is only one correct answer for each 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?  
 (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:
B