动词
中文翻译：变硬
同义词：
['harden', 'stiffen', 'harden up']
例句：
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1.The frost has hardened the ground into a crust.
霜冻使地面变得坚硬如壳。
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2. As the temperature dropped, the leaves began to harden.
随着温度下降，树叶开始变硬。
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3. The old man's hands had hardened from years of manual labor.
这位老人的双手因多年手工劳动而变硬。