unode of a surface

例句：

1. 1.The surface of the earth is covered in ice and snow.
   地球的表面覆盖着冰和雪。
2. 2. The outer layer of the ocean is made up of salt water.
   海洋的外层由盐水组成。
3. 3. The surface of the lake is smooth and clear.
   湖面光滑透明。