Which sentence in the text helps the reader understand its overall text structure?
The Airport Transit System was created to help transport passengers between terminals in 1993.
People could travel to and from the airport quickly and easily, making it a desirable location.
To aid with the O'Hare Modernization Program, a Sustainable Design Manual was created in 2003.
Over the years, O'Hare International Airport has grown and innovated to manage its expansion.