periodic boundary problem

例句：

1. 1.The weather is periodic in nature.
   天气是周期性的。
2. 2. The growth of the plant is periodic.
   植物的生长是周期性的。
3. 3.The cycle of life is periodic.
   生命的周期是周期性的。
4. 1. The boundary between the two countries is clear.
   两国之间的边界清晰可见。
5. 2. The boundary of the park is marked with signs.
   公园的边界被标志牌标出。
6. 3. The boundary of the lake is visible from the shore.
   湖的边界从岸边可见。