easy to shell

例句：

1. 1. The crab has a hard, protective shell.
   螃蟹有硬的、保护性的壳。
2. 2. The oyster's shell is made of calcium carbonate.
   牡蛎的壳由碳酸钙制成。
3. 3. The shellfish industry is important for the economy.
   贝类产业对经济很重要。