Are you sure that the answer is E? I am quite confused.

However, it makes sense as 'OR' is in the middle of two '+ing' words just after a preposition. It's testing PARALLELISM.

My answer was A, though!

damaged and slowed - can never be nouns, they could be adjectives. for example slowed movement or damaged car.

As for gerund, they are always end with -ing
see below:

In English, the gerund is identical in form to the present participle (ending in -ing) and can behave as a verb within a clause (so that it may be modified by an adverb or have an object), but the clause as a whole (sometimes consisting of only one word, the gerund itself) acts as a noun within the larger sentence. For example: Eating this cake is easy.
In "Eating this cake is easy," "eating this cake," although traditionally known as a phrase, is referred to as a non-finite clause in modern linguistics. "Eating" is the verb in the clause, while "this cake" is the object of the verb. "Eating this cake" acts as a noun phrase within the sentence as a whole, though; the subject of the sentence is the non-finite clause, specifically eating.
Other examples of the gerund:
I like swimming. (direct object)
Swimming is fun. (subject)
Not all nouns that are identical in form to the present participle are gerunds.[3] The formal distinction is that a gerund is a verbal noun – a noun derived from a verb that retains verb characteristics, that functions simultaneously as a noun and a verb, while other nouns in the form of the present participle (ending in -ing) are deverbal nouns, which function as common nouns, not as verbs at all. Compare:
I like fencing. (gerund, an activity, could be replaced with "to fence")
The white fencing adds to the character of the neighborhood. (deverbal, could be replaced with an object such as "bench")
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There is no consensus on what role, if any, is played by acid rain in slowing the growth or damaging forests in the eastern United States.
(A) slowing the growth or damaging - parallelism test; see my note
(B) the damage or the slowing of the growth of - damage /slowing are not parallel
(C) the damage to or the slowness of the growth of - simple present tense
(D) damaged or slowed growth of - past tense
(E) damaging or slowing the growth of - correct

Note - Underlined should be in present continuous, so the answer is either A or E. In A - 'slowing the growth' and 'damaging forests' are not parallel... to make it parallel, it should be 'slowing the growth of' and 'damaging forests', so answer is E.

pkit , thanks for the prompt answer, this was a doubt i faced at few other places too.
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sorry for that...i was not aware that thrz a "kudos" button

just observed...

many responses that i get on my posts are really helpful to me...

Will A be correct if you say:
(A) slowing the growth [highlight]of(added)[/highlight] or damaging

I even said E but just wondering if above A and E would have been there, then what would have been the correct choice?
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Press +1 Kudos, if you think my post gave u a tiny tip.

I picked A because I thought "(E) damaging or slowing the growth of" is distorting the meaning of the sentence by indicating that its about 'Damaging the growth of forests or slowing the growth of forests'.

It would have been perfect, had it been 'slowing the growth or damaging of'.

What do you guys think?
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good question. A has the following issue.

role played by acid rain in
slowing the growth forests <------ what? where's "of"?
damaging forests

E changes the places of 'damaging' and 'slowing..' to overcome the above issue and, thus, is the correct answer.
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<<editing the post to include additional information >>

i second that.... ofcourse i am not trying to challenge the OA but doesnt E appear to change the meaning similar to
sc-traditinally-39415.html

the answer for the above link is selected based on unambiguous meaning... isint there a similar problem in E in this one?

sc experts.. need ur help on this one... on why the ans is E... and how are the two problems different.. this will help me gear up to face such problems!!

Thanks

The answer is E. issue is parallelism and meaning.
D -adjectives (past participles ) modifying growth not forest. meaning too also changed
What role acid rain plays in damaged forests - that makes it sound like the forests were already damaged by something else, before the acid rain came along. That's not the intention - the intention is to say that the acid rain is (potentially) causing damage - or damaging the forests- ---From stacey (MG instructor)
C- slowness of growth( showing a state of growth not decreasing in rate of growth )
B-comparing complex gerund to damage .not an issue .but meaning still not clear.E is better than B
A- damaging forest ,again adjective(present participle ) modifying forest