extent 表示“程度”，to the extent 意为：达到那种程度。the extent 经常用作先行词，后接定语从句（定语从句所描述的动作达到了“那种程度”）。与其搭配的介词是to，所以在关系代词which 之前自然用to。

先行词是the extent，后跟 to which 引导定语从句，举例：

No one can forecast the extent to which traffic will grow.

没有人能预测将来交通流量会增长到什么程度。

She didn't realize the extent to which she had been distracted. 

她没有意识到她心烦意乱的程度。

The extent to which the Court will modify these doctrines is, as yet, unclear.

法院对这些原则修正的范围现在尚不清楚。

The extent to which women have lost their rights in Iraq is shocking.

伊拉克妇女丧失权力的程度令人震惊。