GMAT Sentence Correction (SC) | Re: The industrial pollutants known as PCB`s were first

Hello Mike,

Could you please explain the usage of past perfect in this sentence in the light of another OG question.His studies of ice-polished rocks in his Alpine home land, far outside the range of present-day glaciers, led Louis Agassiz in 1837 to propose the concept of an age in which great ice sheets had existed in now currently temperate areas.

In both the questions, some studies led to some results. However,in industrial pollutant question usage of past perfect is justified