Detailed Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Problem: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
Solution:
How is population density expressed?