形容词
中文翻译：坚硬的
同义词：
['stiff', 'inflexible', 'unyielding']
例句：
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1. The ground is so hard that it's difficult to walk on.
地面非常硬，难以行走。
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2. The material is rigid enough to resist deformation.
这种材料足够坚硬，能够抵抗变形。
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3. The structure is rigid, which makes it difficult to modify.
结构僵硬，难以修改。