在介词后面的关系代词用which而不能用that。即“介词+which（代物）”逗号后面，指物的关系代词只能用which。有两个定语从句，其中一个关系代词已有which，另一个可以用that。相反，如果其中一个为that，另一个可以用which。

The picture for which he paid $1,000 was once owned by a duke.

他花了1000美元买下的画曾为一名公爵所拥有。

Crusoe's dog, which was very old now, became ill and died.

克鲁索的狗，现在已经很老了，病死了。

Edison built up a factory which produced things that had never been seen before.

爱迪生办了一个工厂，生产过去从未见过的东西。