Which of the following is an example of a natural boundary?

• A. The 49th parallel north.
• B. The Rio Grande.
• C. The Berlin Wall line.
• D. Brasília.

Answer: (B)

Natural boundaries are defined by physical features that separate places, like rivers and mountains, rather than straight lines drawn on a map. The Rio Grande fits this idea because it’s a river that has long served as a boundary between the United States and Mexico in various sections, using a natural landscape as the divider.

The other options don’t fit the idea of a natural boundary. A straight line of latitude, like the 49th parallel north, is a geometric boundary—it’s based on a mathematical line rather than a landscape feature. The Berlin Wall line was a constructed political boundary, created for control and ideology rather than by nature. Brasília is a city, not a boundary at all.