有读者问： 有一道选择题：
(1)I have many books, some of _________are on chemistry.
A. them B. that C. which D. those
我认为应选A，但答案却是C，为什么？
应选C，因为some of which are on chemistry是一个非限制性定语从句，最好不选A，除非另起一句，变作“Some of them are on chemistry.”。
在非限制性定语从句中，类似some of which的结构不乏其例，再如：
(2)Instead of indulging himself in pop music or disco dancing, both of which he loves, he devotes four evenings a week to night school.
(3)My mother has mailed me four airmail packages, two of which I received, and two of which are still missing.
(4)China has thousands of islands, the largest of which is Taiwan.
(5)It burned 1.01 million hectares, nearly 70 per cent of which was covered by forests.（it指一场大火）
请注意of which前的词语有时亦可置于of which之后，如上述句（5）中的nearly 70 per cent亦可放在of which之后，再如：
(6)At the end of last year, the country's foreign debt came to ＄20.6 billion, of which ＄7.6 billion were long-term low-interest loans.
(7)The garden covers an area of 420,000 square metres, of which more than 70,000 are under water.
