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 "what does the average number of individuals per unit of area do?" with the following options: 
- It decreases
 - doesn't exist
 - allow growth
 - remains static
 - competing for resources
 - vary considerably
 - population gravity
 - rapid expansion


 

      vary considerably   