关系代词which 指代 the extent，之前之所以用to， 是因为 to the extent, to some extent 这个搭配，其中介词to 表示“达到某种程度”。

更多例句：

But we seldom realise the extent to which maths has invaded every corner of our lives. 

但是，我们很少意识到的是，数学渗透到我们生活的每个角落的程度。

One thing I have never understood about Chinese parents is the extent to which they will neglect this. 

对于中国的父母们，我从来就理解不了的一点是他们对此事忽视的程度。

TV's influence should be measured not only in terms of immediate change in behavior, but also by the extent to which it develops a certain view of life.

【翻译】电视的影响力不仅要从行为上的直接变化来衡量，还要看它在多大程度上形成了一定的人生观。

关系代词之前使用介词的思路和方法：

1. 依据与先行词搭配的具体意义确定使用什么介词

I'll never forget the time during which I was with my friends in the country.

我永远不会忘记和朋友们一起在乡村度过的时光。(during 依据during the time 确定)

I will offer you a better way in which you can do the job.

我要给你提供一个做这项工作较好的办法。(in 依据in a better way 确定)

TV's influence should be measured not only in terms of immediate change in behavior, but also by the extent to which it develops a certain view of life.

电视的影响力不仅要从行为上的直接变化来衡量，还要看它在多大程度上形成了一定的人生观。（to 是根据它跟先行词the extent 关系确定的：to the extent）

2. 依据定语从句中动词或形容词的搭配来确定介词

She bought several clothes, on which she spent all her money.

她买了几件衣服，这些衣服花光了她所有的钱。（on 依据spend…on 确定）

It is the grammar rules of which I am not sure.

我没有把握的就是语法规则。（of依据be sure of确定）

3. 根据定语从句所表达的具体意思来确定介词

Can you see the river across which there is a bridge?

你能看见上面有座桥的那条河？

He is swimming in the river, on which there is a boat.

他在那条河里游泳，河面上有一只小船。

Here is the river in which I used to swim.

这儿就是我过去常在里面游泳的那条河。