有读者问： 有这样一句话：
(1)We are not conscious of the extent to which work provides the psychological satisfaction that can make the difference between a full and an empty life.
为什么一定要用to which，用其他如of which, for which不行吗？
这里之所以用to which，是因为关系代词which的先行词是extent，而extent（程度）则常与介词to搭配，用以表达“到……程度”，例如：
(2)The door opened to the extent of its short tight chain.
(3)A computer is intelligent only to the extent that it can store information.
the extent to which常见于正式或书面语中，再如：
(4)The extent to which you had outstripped him hurt him.
(5)The measure of success of the Revolution was largely to be found in the extent to which it boosted production and enabled the families to live a better, more comfortable life.
关系代词之前亦可用其他介词。但用什么介词则决定于其前的先行词的意义如何。现仅举几例如下：
(6)I am aware there is always a limit beyond which I cannot go, but one thing is certain: be the best of whatever you are. （which的先行词是a limit, beyond与limit搭配）
(7)The picture for which you are looking is in the drawer.
（which的先行词是the picture, for与look搭配，构成短语动词）
(8)Tell me the time at which you'll be ready.
（which的先行词是the time, at与time搭配）
(9)This is the house in which Lu Xun once lived.
（which的先行词是the house, in表“在……内”）
(10)The old house about which I was telling you is empty.
（which的先行词是the old house, about表“关于”）
(11)That was the period through which she lived here.
（which的先行词是the period, through表“贯穿”）
