Fact 1: Population density is the average number of individuals per unit of area or volume.

Fact 2: Population density varies considerably.

Given the two facts above, answer the question "the average number of individuals per unit of area or what can vary considerably?" with the following options: 
- Decibels
 - colors
 - dense
 - Summer
 - volume
 - seasons
 - animals
 - decreases


 

     volume    