Some soil scientists have asserted that decaying matter on the forest

this passage says that there are two sources that cause acidity in mountain lakes :
(i) decaying matter on the forest floor
(ii) acid rain

The passage also lets us know that the 'decaying matter' causes far greater acidity than the acid rain itself. The passage then goes on to make the conclusion that 'eliminating acid rain WILL NOT reduce the acid levels of mountain lakes'.

To weaken this conclusion, we need to come up with an option that shows that "acid rain certainly affects the acidity levels of mountain lakes to some great extent, one way or the other".


Now, let us look for an answer choice that does exactly this.

(A) It is natural for mountain lakes to have acidity levels higher than those of other lakes. - knowing (A) as a fact does nothing to weaken the idea that 'acid rain reduction does not help reduce the acid levels'. Hence, (A) is incorrect.

(B) The harmful effects of increased acidity levels in lakes have been greatly underestimated. - Even after considering this as true, it still does nothing to weaken the idea that elimination acid rain will have the intended benefits (as described in the conclusion). hence, (B) is incorrect. So, eliminate (B).

(C) Acid rain is found in urban and heavily industrialized regions of the country. - acid rain in urban and heavily industrialized areas is not related to what happens in the mountain lakes (in the context of this passage). (C) is therefore irrelevant. Hence, Eliminate (C)

(D) There is much disagreement among soil scientists about the causes of acid rain. - A simple disagreement (between scientists) still does nothing to weaken the conclusion. hence, Eliminate (D).

(E) While plant life remains, acid rain significantly increases the amount of decaying organic matter in natural environments. - This is the most interesting answer choice among the list. We already know that existing decaying matter on forest floor is a source that causes acidity in the mountain lakes. If acid rain increases the amount of such decaying matter, then the quantity of the source of acidity increases. When the amount of acidity source increases, the acidity levels at the mountain lakes are bound to increase further. Hence, by eliminating acid rain, we will be able to stop the growth of additional decaying matter on the forest floor, thereby preventing a further increase in the acidity levels of mountain lakes. This weakens the conclusion that 'reducing acid rain doesn't reduce the acid levels of mountain lakes. Hence, (E) is the correct answer.