填什么?They increased restriction on the type of material （）dum

这句话在没有上下文的情况下有多种表达：
They increased restriction on the type of material （dumped）.
They increased restriction on the type of material（ being dumped）.
They increased restriction on the type of material（to be dumped）.
说明：括号内均为后置定语,表被动,限定前面的名词短语the type of material,分别表示 dump 已发生、在发生、将发生；
如果非得加 been ,句子会做调整如下：
They increased restriction on the type of material that had been dumped.