Use the text below to answer the following question:

Case study: The Very Big Apple
With over eight million people, New York City is the most heavily populated city in the U.S. Between 1800 and 1900, the population of New York increased from about 80,000 to over three million people. In the years after the Civil War, the population of New York City tripled. With a large influx of European immigrants New York became known as the "melting pot." New York has always had the highest population density of any U.S. city. According to the 2000 census, New York City has about 26,403 people per square mile—almost twice the number of people per mile as Chicago.

What can you determine was a major issue for people living in New York City in the mid- to late 1800s?

> Education
> Recreation
> Sanitation
> Transportation