把面团放进冰箱后，它会慢慢变硬。

The dough ___hardens slowly when you put it in the fridge.

多年的军旅生活让他在情感上变得坚强。

Years in the military ___hardened him emotionally.

学校对迟到政策变得更加严格了。

The school ___hardened its policy on tardiness.