你刚刚塞到塑料杯子里面的是神马东东?
which是关系代词,引导定语从句,修饰并指代that.主句是What is that?which 能用哪个词代替？which不能代替。本来这样的定语从句which和that可以互换的，但如果先行词是that，再用that引导定语从句就会很怪，所以只适合用which.