Why Does the Intensity of Waves from a Small Source

Why does the intensity of waves from a small source decrease with the square of the distance from the source?

A) The frequency of the waves decreases as they get farther from the source.
B) The waves run out of energy as they travel.
C) The medium through which the waves travel absorbs the energy of the waves.
D) The waves slow down as they travel away from the source.
E) The waves spread out as they travel.