Answer the following question given this paragraph: 

Ecologists look at many factors that help to describe a population. First, ecologists can measure the number of individuals that make up the population, known as population size . They can then determine the population density , which is the number of individuals of the same species in an area. Population density can be expressed as number per area , such as 20 mice/acre, or 50 rabbits/square mile.


Q: How is population density expressed?

 Choices:

- per arable land

- number per household

- arable land per capita

- number per area

A:
number per area
