shell

我在森林小径上发现了一个小小的蜗牛壳。

I found a tiny snail ___shell on the forest path.

没有坚果钳，很难敲开巴西坚果的壳。

It's hard to crack the ___shell of a Brazil nut without a nutcracker.

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.