The Galactic Habitable Zone Is Sometimes Defined as a Torus

The galactic habitable zone is sometimes defined as a torus around the galactic center with an outer radius of 10 kpc and an inner radius of 4 kpc. The volume of such a torus is roughly 5600 (kly) ³. Adopting the text's value of N = 100 technologically advanced civilizations in the Galaxy, then, on the average, each volume of 56 (kly) ³ within the galactic habitable zone is occupied by one such civilization. Also adopting the value L = 500 years as the technological lifetime of such a civilization (the time over which they have been sending out radio signals at the speed of light) , then through what volume of space would these signals have spread? What are the chances of two civilizations being within the same signal volume?

A) The volume of signals is 0.01 (kly) ³, so the chances of two civilizations being within the same signal volume are very small.
B) The volume of signals is 0.5 (kly) ³, so the chances of two civilizations being within the same signal volume are very small.
C) The volume of signals is 50 (kly) ³, so the chances of two civilizations being within the same signal volume are reasonably good.
D) The volume of signals is 500 (kly) ³, so the chances of two civilizations being within the same signal volume are very great.