boundary of a set of points

例句：

1. 1. The boundary between the two countriesis marked by a river.
   两国之间的边界由一条河流划分。
2. 2. The boundary of thepark is clearly marked with signs.
   公园的边界被清晰的标志标出。
3. 3. The boundaryof the forest was difficult to navigate.
   森林的边界难以辨认。