According to the U.S. Census Bureau, Philadelphia had a population density of 11,379.5 people per square mile in 2010, while the entire U.S. had a population density of 87.4 people per square mile. Which best explains the contrast between these two population densities?

While most areas of the U.S. have a high population density, the U.S. encompasses a vast amount of land.

The birthrate decreased throughout the U.S.

While some areas of the U.S. have a high population density, others must have extremely low population densities.

All of the above are correct.