Dear friends and moderators,

Although ice particles in the upper atmosphere benefit Earth in that they reflect and absorb solar radiation, acting as global thermostat and thus keeping Earth from either burning up or freezing over, they also accelerate the destruction of the ozone layer by reacting with chloroflurocarbons (CFC's).

A) acting as global thermostat and thus keeping Earth from either burning up or freezing over, they also accelerate
B) acting as a global thermostat and thus keeping Earth either from burning up or freezing over, while also accelerating
C) act as a global thermostat and thus keep Earth from either burning up or freezing over, while also accelerating
D) they act as a global thermostat that thus keeps the Earth either from burning up or freezing over, even though it also accelerates
E) they act as a global thermostat to thus keep Earth from either burning up or freezing over, but they also accelerate

OA: A

Can I check my reasoning with you guys?

For älthough", we need to note the shift in the meaning in the sentence, and in this question, the shift comes ".....they also accelerate". Thus DE is out.'

C - äct" behaves likes a run on sentence..."while" is unnecessary due to the presence of älthough".

B - error in ëither from", unnecessary "while".

Correct?

Thanks
KH

I believe that there are more errors.
BD: "from burning up" and "freezing over" are not parallel.
BC: no main clause.

B) acting as a global thermostat and thus keeping Earth either from burning up or freezing over, while also accelerating
C) act as a global thermostat and thus keep Earth from either burning up or freezing over, while also accelerating

only these two options qualify for the ansers..other are having ambiguity/mismatch with IT and THEY.

from these two, B) should be the correct ans..becasue the the ICE does work in two ways 1) acting as... 2) accelerating..

one important point that posters have so far missed: 'although' indicates contrast. therefore, if the basic structure of a sentence is 'Although X, Y' (where X and Y are clauses), then X and Y must, in some sense, stand in opposition to one another. this fact rules out choices d and e, because the 'global thermostat' idea flows from the ice particles' reflection and absorption of solar radiation.

choice c is out because it makes no grammatical sense. in fact, i can't even tell which correct form it's supposed to be a corruption of; my best guess is that it's supposed to be a list (x, y, and z) with terrible parallelism: reflect ..., act ..., while also accelerating ....

that leaves a and b.

b is a sentence fragment: it has no main clause. it consists of one subordinate clause followed by a wealth of modifiers. in addition, it contains faulty parallelism (either from burning up or freezing over).

--

choice a:
* first off, i'll assume that you meant to write acting as a global thermostat in choice a. i've never seen the gmat omit an article, even in an incorrect answer choice.
* the adverbial modifier (acting as ...) correctly modifies the clause about the ice particles' reflection and absorption of solar radiation, because that modifier describes the results of those phenomena.
* the main clause (they also accelerate) starts in the correct place, and, as expected from although, stands in conceptual opposition to the subordinate clause that begins the sentence: it describes deleterious effects of the ice particles, while the subordinate clause describes beneficial effects.

Hei Ron

Thank you for your wonderful explanation.
I have a question about choice A.
Is there a 'squinting modifier' within the choice A above?

I found an example as follows

(deleted - you can't post official guide problems here; that's a banned source - sorry!)

which part of choice (a) do you think is 'squinting'? i don't see it, but maybe i'll see what you mean if you point it out.

Hi, Ron

I mean the 'acting' part, I think 'acting' can also modifiy 'radiation' ambiguously, does it?

nope.
if you have a present participial (-ing) modifier that follows a comma (as does this one), it is unambiguously an adverbial modifier, which modifies the action of the preceding clause.

if there is no comma, then it's an adjective modifier that is committed to the immediately preceding noun.

examples:

james mistakenly turned off the highway, heading toward louisville --> adverbial modifier.
this sentence means that james wound up going to louisville because of the wrong turn he made (i.e., he meant to go to, say, lexington instead).

james mistakenly turned off the highway heading toward louisville --> adjective modifier.
this sentence means that james didn't go to louisville, because he accidentally turned off the highway that goes to louisville.

Thank you Ron.
I got it.

But I m sorry to bother you again about the choice A.
Can we regard the 'acting' part in choice A as an adverbial modifier of the clause that follows it after the comma?

I mean 'Althoug..., acting..., they also accelerate...'
Is it a 'squinting modifier'?

Thanks again

well...
i can see what you're saying - technically, you could read the modifier in either of those two ways.
so:
this is where i break out what i like to call the 'all purpose backup response', which is the following:
you have just learned something about the rules that the gmat follows.
acording to their rules, then, modifiers written in this sort of way are ok. store that fact in your memory for when you take the test!

by the way, i'm assuming that choice (a) was meant to say, 'acting as a global thermostat'; you forgot to type the 'a'. in fact, it's missing from the original and from choice (a), but i'll assume that you just cut and pasted it.

Hi Ron

I see the 'a' in choice (a).
thank you very much!

We're glad it helped!