According to the 2000 census, what was a notable demographic trend in the U.S.?

The correct answer highlights a significant demographic trend in the United States reported by the 2000 census, which was indeed that a substantial portion, around 50% of the population, lived in suburbs. This trend reflects the post-World War II movement of American families from urban centers to suburban areas, a cultural and social shift that accelerated in the latter half of the 20th century. Factors contributing to this suburbanization included the availability of affordable housing, the rise of the automobile, and the desire for more space and a perceived higher quality of life outside crowded cities.

The suburban growth indicates a shift in lifestyle preferences and demographic patterns, illustrating how American society was evolving during this period. The contrast with other options further emphasizes this point, as they suggest lower percentages of urban or rural populations, which do not align with the realities captured in the census data. Thus, option B is consistent with the observed changes in American living arrangements at the turn of the millennium.