which 是主语,因为你可以把先行词放到从句里看,变成the building stands near the river这样就很明显了.顺便说一下,因为做主语,所以which不能省略The room in which there is a machine is a workshop.那in which在句中做什么成分呢，谢谢啊in which 当然是状语啦，都有介词了诺。同样把the room放到从句中看，there is a machine in the room