请找出下面段落中的主题句的位置。 Over the past hundred years, many North Americans have moved from the country to the city. In recent years, people have moved to the suburbs of cities. These moves have caused North American customs to change. One major change has been in shopping habits. Farmers, city dwellers, and people living in the suburbs all have their own special approaches to buying things. Depending on where they live and the kind of shops available to them, North Americans today have three different ways of buying what they need.