of後面加which的時候是which後面要帶一個從句修飾前面的名詞。加them就是指代前面所有的了，不需要加從句。

在定語從句中，of 後面是which，在並列句中，of後面是them.，例如：

He wrote many children's books， nearly half of which were published in the 1990s.他寫了很多兒童讀物，其中將近一半是在二十世紀九十年代出版的（本句中，逗號前後各有一個句子，因為逗號不能其連線作用，所以用which來連線，成為定語從句的關係代詞，因此說在定語從句中，of後是which）

He wrote many children's books and nearly half of them were published in the 1990s.本句是and連線的並列句，所以 of後面是them。