英语题，定语从句方面的。求详解

这是一种特殊的非限制性定语从句。非限制性定语从句是对先行词（本句中即two sons）提供进一步的信息。
1. who是主格代词，只能在从句中作主语，如：
He has two sons, who work as chemists.
2. 在both of结构中，因of是介词，介词后面的代词必须是宾格，whom就是宾格。
He has two sons, both of whom are chemists. (正确答案）
3. which即是主格也是宾格，但是它是代事物, 不可代人，如：
I have two toy cars, both of which are made in China.
4. all作为不定代词是代三个以上，比如：
He has 3 sons, all of whom are chemists.
5. two of whom在此句中显得语无伦次。数量词+of whom/which的含义是“其中x个”，"他有两个儿子，其中两个是化学家”，显然不合理。但是先行词中的数量增加到3个以上，就可以了：
He has 3 sons, two of whom are chemists. 他有三个儿子，其中两个是化学家。
扩展：
在这种定语从句中，除了both, all外，还有更多表示数量的不定代词或数词：
He has 2 sons, neither of who are chemists. (neither两者都不）
He has 3 sons, none of whom are chemists. (none三者以上全不）
He has many friends, some of whom are chemists.
He has many friends, most of whom are chemists.
He has many friends, a majority of whom are chemists.
He has many friends, half of whom are chemists.
He has many friends, few of whom are chemists.
He has many friends, 40% of whom are chemists.
He has many friends, nearly 1/3 of whom are chemists.
祝你进步。

后面是非限制性定语从句，因为谓语动词不是单数第三人称形式，所以关系代词应该选择 B 指代复数宾语 two sons，即 He has two sons, both of whom work as chemists.

A、(意思不一定是2个)，C、(Which是指代物)D，(all是用于三者以上)，因此选B。

who 两个儿子，定语从句中缺主语，填Who

B
sons 为人，排除C
只有两个儿子，排除D
既然只有两个，A也排除了。