The correct answer is
C. reduce the friction between its moving parts
In fact, by reducing the friction between the moving parts of the machine, it is possible to reduce the energy wasted due to this friction; therefore, more input energy is converted into useful work, and this will improve the efficiency of the machine.
That particular group of elements is reffered to as the "Noble Gasses"--a title that comes from the fact that these gases are very "secure" and don't mix well with other elements.
Answer: C
Explanation: Amplitude controls loudness, and frequency controls pitch. The more frequent the higher pitch.
