don't buy cups or boxes which can be used only once.为什么用which?what呢?boxes不是复数吗可用which

don't buy cups or boxes which can be used only once.为什么用which?what呢?boxes不是复数吗可用which

which引导得定语从句修饰前面的box,cups.和box单复无关.what不能用来引导定从,一般用来引导主从

which后面的是定语从句，修饰cups or boxes
也可以用that
不能用what