Seems to be a bit of back and forth on this, so I thought I'd try to take it on using the (dreaded) process. I know, I know. Nothing's less fun then taking notes...but let's do it anyway:
Conclusion: Major cause of decline in LC was decline in lead gas [It's pretty common to find the conclusion in the question stem...so always be on the lookout]
Premises: Decline between 75 and 85. Feds said industrial discharge down in 1975. 75% drop in lead gasoline between 75 and 85.
Assumption: Industrial discharge didn't do it. [Basically, we're looking for something to downplay the importance of the legislation.]
(A) The level of lead contamination in United States rivers fell sharply in both 1975 and 1983.
PROBLEM: This would possibly weaken. We wouldn't want to see sharp declines, because it sounds like the drop was gradual (as lead gas was phased out).
(B) Most of the decline in industrial discharges of lead occurred before 1976, but the largest decline in the level of river contamination occurred between 1980 and 1985.
ANSWER: Well, if the industrial decline occurred before 1976, that means that the effect of the legislation was almost immediate. But if the contamination dropped mostly between 80 and 85, that could only be because of the rise of unleaded gas.
(C) Levels of lead contamination in rivers fell sharply in 1975-1976 and rose very slightly over the next nine years.
PROBLEM: This weakens a lot. This sounds like the legislation had a big effect, and then nothing happened for the rest of the time. We wanted gas to slowly make things better, but they're actually getting worse!
(D) Levels of lead contamination rose in those rivers where there was reduced river flow due to drought.
PROBLEM: This doesn't connect to either industry or gas, so it's irrelevant.
(E) Although the use of leaded gasoline declined 75 percent between 1975 and 1985, 80 percent of the decline took place in 1985.
PROBLEM: Alright. Here's the toughie. The problem is that this doesn't actually add any information that we didn't have in the passage itself. Looks like it does, doesn't it? But think about it. We already knew the use of leaded gasoline declined 75 percent between 75 and 85 (that part is LITERALLY in the passage word for word). Then we're just told that most of it happened at the end. So what? That doesn't tell us anything about the contamination itself. What we really wanted was to WEAKEN the influence of the legislation, which B does.
Hope that helps!
Tommy Wallach | Manhattan GMAT Instructor | San Francisco
Manhattan GMAT Discount | Manhattan GMAT Reviews