The key question of what caused the leak remains unanswered.

泄漏究竟是怎么造成的，这一关键问题仍未得到答案。

At the heart of the debate over illegal immigration lies one key question: are immigrants good or bad for the economy?

关于非法移民的争论的核心是一个关键问题：移民对经济是好还是坏？

But he added that the overall research in this area is "still in its infancy" and one key question is whether literary fiction really is better than other fiction.

但他补充说，该领域的整体研究“仍处于起步阶段”，一个关键问题是，文学小说是否真的比其他小说更好。

Now comes a key question.

现在有一个关键问题。

The key question is what to do when.

关键问题是什么时候应该用什么。