巴西坚果的外壳极其坚硬，很难敲开。

The ___shell of a Brazil nut is extremely hard and difficult to crack open.

她在沙滩上收集了各种形状和大小的美丽海贝壳。

She collected beautiful sea ___shells of all shapes and sizes along the sandy beach.

为了保护自己，蜗牛可以将柔软的躯体完全缩回坚硬的壳里。

For protection, a snail can fully retract its soft body into its hard ___shell.