The level of lead contamination in United States rivers

Questions:

In selecting B, you're making an assumption that the drop in industrial lead discharge results in an immediate drop in lead contamination level.

Secondly, you're making an assumption, that the drop in use of leaded gasoline happened primarily after 1976. This isn't mentioned - all they mention is that the use of leaded gasoline dropped 75% between 75 and 85.

Without a doubt B.
It is the most relevant argument supporting the claim made.

B for me. It shows that the law enacted in 1975 did not have as much effect on the decline. The steep drop between 1980 and 1985 suggest it took a while before the drop in the use of leaded gasoline has an effect on the level of lead contamination in U.S rivers.

In selecting B, you're making an assumption that the drop in industrial lead discharge results in an immediate drop in lead contamination level.

--> I think selecting B does not require an assumption that the drop in industrial lead discharge results in an immediate drop in lead contamination level. We're already told in B, that the biggest decline only came later in 1980 to 1985. This suggests that the improvement is a gradual process.

Secondly, you're making an assumption, that the drop in use of leaded gasoline happened primarily after 1976. This isn't mentioned - all they mention is that the use of leaded gasoline dropped 75% between 75 and 85.

--> We're told in the passage the use of leaded gasoline came between 75 and 85. The drop in the use was 75%. Since pre-75' there was a law and the contamination levels didn't really improve, then the improvement should have come from the declining use of leaded gasoline. No assumption is needed.

In any case, the remaining choices are just plain bad. What do you think should be the answer if you think B is a bad choice?

B for me too.

you all are CR kings
OA is B.

All the other answer choices are bad. Where did you get this CR?

B is an easy choice

Will go with B too.
If the industrial discharge declined before 1976, then the possible cause for the decline in river contamination bt 1975 and 85 would be the decline in use of leaded gasoline

Its amazing none of the guys even considered E. I must really suck at CR, I guess!

The level of lead contamination in United States rivers declined between 1975 and 1985. Federal regulations requiring a drop in industrial discharges of lead went into effect in 1975, but the major cause of the decline was a 75 percent drop in the use of leaded gasoline between 1975 and 1985.
Which of the following, if true, best supports the claim that the major cause of the decline in the level of lead contamination in United States rives was the decline in the use of leaded gasoline?
(A) The level of lead contamination in United States rivers fell sharply in both 1975 and 1983.
This does not explain the intended reason

(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.
This one is the correct choice, it correctly says that the majot decline was not because of drop in Industrail waste discharge

(C) Levels of lead contamination in rivers fell sharply in 1975-1976 and rose very slightly over the next nine years.
This says the opposite that the main reason was drop in Industrail waste discharge

(D) Levels of lead contamination rose in those rivers where there was reduced river flow due to drought.
Irrelevant

(E) Although the use of leaded gasoline declined 75 percent between 1975 and 1985, 80 percent of the decline took place in 1985.
DEcrease could be because of any reason, so this one is ruled out

B.