根据以下资料，回答20~23题。 There are many stereotypes about the character of people in various parts of the United States．In the Northeast and Midwest, people are said to be closed and private．In the South and West, however, they are often thought of as being more open and hospitable．Ask someone from St．Louis where the nearest sandwich shop is, and he or she will politely give you directions．A New Yorker might eye you at first and after deciding it is safe to talk to you, might give you a rather unexpected explanation．A person from Georgia might be very kind about directing you and even suggest some different places to eat．A Texan just might take you to the place and treat you to lunch. American stereotypes are abundant．New Englanders are often thought of as being friendly and helpful．Southerners are known for their hospitality and warmth．People from the western part of the United States are often considered very outgoing．These differences in character can be traced to different factors such as climate, living conditions, and historical development. When travelling from place to place, Americans themselves are often surprised at the differing degrees of friendliness in the United States. The main idea of the passage is __.