这块木头的硬度让它很难被切割。

The ___hardness of the wood makes it difficult to cut.

这道数学题的难度让全班同学都感到头疼。

The ___hardness of this math problem frustrated the whole class.

他批评下属时语气中的严厉让人难以接受。

The ___hardness in his tone when criticizing his staff was hard to accept.