桦树的树皮是白色的。

The ___bark of the birch tree is white.

史前人类通过有节奏地敲击树皮来进行交流。

By hitting the ___bark of trees rhythmically, prehistoric humans communicated.

厚厚的树皮保护树木免受森林火灾的侵害。

The thick ___bark protects the tree from forest fires.