再问指代问题，prep 19

Gail小M

songlovegt is right, --they were developed-they refer to "pollutants".

drzard

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gastank

Industrialization and modern methods

潇湘妃子zwhj

如果句子的意思是，这些methods have+done... while doing... that 修饰 chemical pollutants，而且最后说到，gone virtually unregulated，那么全句的现在完成时have done就正好需要一个相应的时间词since，既然since是配合have done的，那么they必然是指methods的。
跟while那句话没有关系，跟while里面的that定语从句就更没关系了。