GMAT Sentence Correction (SC) | Re: Industrialization and modern methods of insect control have

Industrialization and modern methods of insect control have improved the standard of living around the globewhile at the same time they have introduced some 100,000 dangerous chemical pollutants, having gone virtually unregulated since they were developed more than 50 years ago.
A. while at the same time they have introduced some 100,000 dangerous chemical pollutants, having
B. while at the same time introducing some 100,000 dangerous chemical pollutants that have
C. while they have introduced