老师你好，今天看到一个题目：
In the corner of my yard stand two trees, both of ____ are jujubetrees. I have a lot of dates to eat during the harvest time.
我看答案给的是which,而不可以用them...
我能理解为，这里是同一个句子，which是引导先行词tress的关系词，形成一个定语从句吗？both of 还可以加在定语从句的关系词前么?了解可以加介词，但both of 也可以么？
《牛津英语使用方法指南》第四版237.9指出：“在非限制性从句中，of whom, of which 及 of whose 可以和数目限定词（如 some, any, none,all, both, several, enough, many, few）连用。这种限定词一般放在 of which / whom /whose 前面，但在很正式的文体中，有时也可以放在其后面。”
所以，网友那道题的答案是正确的。假如用 them，句子应该是：
In the corner of my yard stand two trees, and both of them are jujubetrees. I have a lot of dates to eat during the harvest time.
