B
Below is a question with multiple-choice answers. Choose the correct option based on your reasoning.

### 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?

### Choices:
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:
