Given a scientific passage and an answer, generate a question for the given answer.

Let me give you an example: Passage: A frameshift mutation is a deletion or insertion of one or more nucleotides that changes the reading frame of the base sequence. Deletions remove nucleotides, and insertions add nucleotides. Consider the following sequence of bases in RNA:. Answer: nucleotides.
The answer to this example can be: A frameshift mutation is a deletion or insertion of one or more of what that changes the reading frame of the base sequence?
Here is why: The science passage defines frame mutation and the main compoment here is nucleotides. Hence the generated question is apt.

OK. solve this:
Passage: 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. Answer: number per area
Answer:
How is population density expressed?