小朋友,如果你打开世界地图,看看美国和加拿大的边界线,就会发现一个有趣的现象。别的国家边界线,由于地形地貌、人口分布等原因,划分时都是随之变化,弯弯曲曲。独有美加边界线,如刀切一般整齐,这是为什么呢? Children, if you open a map of the world and look at the border between the United States and Canada, you will find an interesting phenomenon. Other countries borderline, due to topography, population distribution and other reasons, when the division is followed by changes, winding. Unique US-Canadian border line, such as knife cut generally neat, which is why?