Why Would You Use Friiss's Formula

Why would you use Friiss's formula?

A) To provide the overall noise effect of a multistage system.
B) To break down any periodic waveform into a series of sine and/or cosine waves at multiples of the fundamental frequency.
C) To establish that different users can share bands of frequencies provided that each user was assigned exclusive use of a frequency range within that band.
D) To show how the construction of a square wave can be seen to be made up of a number of sinusoids.