一道模考题！

 Modern methods have improved living while at the same time they have introduced pollutants, having gone unregulated since 50 years ago. 

A 原文

B. while at the same time introducing pollutants that have

C. while they have inroduced pollutants at the same time, which have

想问两个问题，1。在这里是用introducing和they have introduced都可以么？

                        2。如果都是紧跟修饰词，用which have有错么？谢谢