资料题：What is the third special thing about a city（　）。

A city is one kind of community.There are three special things about a city.A city is a community with a large population.That is the first special thing.The population of a place is the number of people who live there.A big city may have millions of people.A small city may have only twenty thousand.That is small for a city, but it is still a large population.The second special thing about a city is that it has a large area.The amount of land a community covers is its area.A city takes up a lot of land.Cities are crowded.That is the third thing that makes cities special.In large cities, there are thousands and thousands of buildings.The buildings are very close together.Many people live, work, or shop in each building.All these people use the streets and sidewalks.They use the parks and playgrounds.City people are crowded close together.