For this question, we are trying to find the answer choice that would provide a plausible explanation for why there is a lower recorded level of oil contamination and PAHs during the war despite many oil fires and spills compared to before the war.
In this sense, I would predict that we need some new piece of information that would show that, before the war, there was even more oil contamination and PAHs for some logical reason.
Answer choice D gives us this piece of information in a concise manner. It states that before the war, oil production and transport resulted in very high levels of contamination and spills. This would explain why during the war, when oil production slowed down, there would be less oil contamination and levels of PAH. It seems that even with the many spills and fires during the war, this was not enough to compensate for the massive contamination and spills that occur during full-steam production.
Answer choice E tells us when the Persian Gulf War ended relative to the expectation of how much damage was thought would occur. So to put this into perspective, it may have been expected that people thought there would 100% damage, but the war ended when there was 80% damage. These numbers are arbitrary, they are simply serving the purpose to show that the war ended before it did the damage people thought it would. But how does this help explain that despite the many spills and oil fires, there was a lower level of contamination and PAHs? It doesn't. It just gives a timeline relative to people's expectations of damage.