请问下这个句子的结构 Societies offer a great chance to meet people from outside college,which...请问下这个句子的结构 Societies offer a great chance to meet people from outside college,which,especially for arts students,is something it is sometimes difficult to do .我知道这是一个非限制性定语从句,especially for arts students 是一个插入语,但是后面的 is something it is sometimes何解?it is sometimes 在句子中做何成分,

请问下这个句子的结构 Societies offer a great chance to meet people from outside college,which...
请问下这个句子的结构 Societies offer a great chance to meet people from outside college,which,especially for arts students,is something it is sometimes difficult to do .我知道这是一个非限制性定语从句,especially for arts students 是一个插入语,但是后面的 is something it is sometimes何解?it is sometimes 在句子中做何成分,