吃透英语课本，提升在校竞争力
v. 变坚韧；变顽强；变强硬；变困难
现在分词：toughening
第三人称单数：toughens
过去式：toughened
过去分词：toughened
Nevertheless, researchers have discovered that a process long known to discolor and toughen foods may also contribute to age related impairment of both cells and tissues.
然而，研究人员发现，一个长期以来众所周知的使食物变色和变硬的过程也可能与年龄相关的细胞和组织损伤有关。
She worked hard to toughen her resolve.
她努力使自己更加坚定。
Regular exercise will toughen your muscles.
定期锻炼会增强你的肌肉。
