tough outer covering for the fruit

例句：

1. 1.The tough outer covering of the apple is what makes it so delicious.
   苹果的外层硬皮是它如此美味的原因。
2. 2. The skin of the orange is tough and can be difficult to peel.
   橙子的外皮很硬，很难剥开。
3. 3.The rind of the lemon is tough and can be a bit bitter.
   柠檬的外皮很硬，有点苦味。