there are three libraries in our school,all of which were built three years ago.
问题描述：
there are three libraries in our school,all of which were built three years ago.
如果把句子里的 all of which 改成“and all of them
答
这个是可以的
there are three libraries in our school这句已经是完整的一句了,所以只能用关系代词来连接
但如果加and 的话all of them也行.就是两个句子并列了~
你可能会在选择题中碰到过all of them不加and 就不对了，是吗？是的，如果不加and,这个句子就有两个谓语，所以必须用关系代词连接的~~~