pivotal

该原则对解决这一悖论至关重要。

The principle is ___pivotal for resolving the paradox.

她四十岁时决定学习普通话后来证明是关键转折点，开启了横跨上海和新加坡的第二职业生涯。

Her decision to learn Mandarin at age forty proved ___pivotal, leading to a second career that spanned Shanghai and Singapore.

天花疫苗被证明有效的那一刻是人类历史上的关键节点，拯救了无数生命。

The moment the smallpox vaccine was proven effective was ___pivotal in human history, saving countless millions of lives.