HOL

例句：

1. 1. The hollow tree is a symbol of nature's beauty.
   空心树是大自然美丽的象征。
2. 2. Hollow core technology is used in the production of high-strength concrete.
   空心核心技术用于生产高强度混凝土。
3. 3. The hollow space between the two plates is filled with oil.
   两块板之间的空心空间被油填满。