Cities develop as a result of functions that they can perform, some functions result directly from the ingenuity of the citizenry, but most functions result from the needs of the local area and of the surrounding hinterland (the region that supplies goods to the city and to which the city furnishes services and other goods). Geographers often make a distinction between the situation and the site of a city. Situation refers to the general position in relation to the surrounding region, whereas site involves physical characteristics of the specific location. Situation is normally much more important to the continuing prosperity of a city. If a city is well situated in regard to its hinterland, its development is much more likely to continue. Chicago, for example, possesses an almost unparalleled situation: it is located at the southern end of a huge lake that forces east-west transportation lines to be compressed into its vicinity, and at a meeting of significant land and water transport routes. It also overlooks what is one of the world's finest large farming regions. These factors ensured that Chicago would become a great city regardless of the disadvantageous characteristics of the available site, such as being prone to flooding during thunderstorm activity.
Similarly, it can be argued that much of New York City's importance stems from its early and continuing advantage of situation. Philadelphia and Boston both originated at about the same time as New York and shared New York's location at the western end of one of the world's most important oceanic trade mutes, but only New York possesses an easy-access functional connection (the Hudson-Mohawk lowland) to the vast Midwestern hinterland. This account does not alone explain New York's primacy, but it does include several important factors. Among the many aspects of Situation that help to explain why some cities grow and others do not, original location on a navigable waterway seems particularly applicable. Of course, such characteristic as slope, drainage, power resources, river crossings, coastal shapes, and other physical characteristics help to determine city location, but such factors am normally more significant in early stages of city development than later.
What does the passage mainly discuss？

A. The development of trade mutes through United States cities
B. Contrasts in settlement patterns in United States
C. Historical differences among three large United States cities
D. The importance of geographical situation in the growth of United States cities