To meet the rising marketing demand for fish and seafood, suppliers are growing fish twice as fast as their natural growth rate, cutting their feed allotment by nearly half and raising them on special diets.
A. their natural growth rate, cutting their feed allotment
B. their natural growth rate, their feed allotment cut
C. growing them naturally, cutting their feed allotment
D. they grow naturally, cutting their feed allotment
E. they grow naturally, with their feed allotment cut

How to eliminate between C and D?

And also in D they refers to what suppliers or fish and seafood . Why I am asking is in non-underlined portion raising them refers to what ?

This is a GMAT prep question

you have a point with the pronoun reference: technically, it's ambiguous (although the intended meaning is pretty clear). however, since the same 'problem' is present in all five of the answer choices, i guess we shouldn't consider it a problem.

in answer choice c, 'growing them naturally' appears to be parallel to 'growing fish twice as fast...' therefore, the default interpretation of this sentence is that the suppliers are growing the fish (at 2x the normal rate), yet somehow also growing the fish naturally (at the normal rate). not only is this nonsensical, but it alters the meaning of the original sentence.

i agree with you that 'they' is a problem.

Hi Ron

As per answer D is correct . Really confused with this sentence

Thanks
Vik

Ron already addressed the issue of why C is wrong based on its meaning. I'll try to tackle the pronoun issue.

ACT / ETS (depending on who wrote this problem) doesn't seem to have a hard-and-fast pronouns rule - sometimes they allow something that seems ambiguous to us. Here's my guess as to why they think this is NOT ambiguous (ie, why they think it's fine).

This sentence has a list of three things: suppliers are A, B, and C.
A = growing
B = cutting
C = raising

A mentions the noun "fish" and then uses the pronoun "they" shortly thereafter. Examine the specific words: "growing fish twice as fast as they grow naturally." We're comparing fish to fish here - it wouldn't make sense to say that we're growing fish twice as fast as suppliers grow naturally - so the "they" here refers to "fish." Fine.

Then we know that A, B, and C need to be parallel, since we have a list. So "cutting their" and "raising them" should refer back to the same noun A was talking about - fish. Do they? Yes, they do, so the test says this is fine.

Also, I just want to echo something Ron said - although we think of this as ambiguous, notice that they don't make us choose based upon this issue. All five choices have the "problem" - so I should just ignore it. Any time you find what you think is a problem but the answer choices ALL contain it, find something else!

In answer choice "C" as you have said that "default interpretation of this sentence is that the suppliers are growing the fish (at 2x the normal rate), yet somehow also growing the fish naturally (at the normal rate)."

What is the reason in choice "C" that this second interpretaion is also valid? Based on what rule/sentence structure this interpretation is done? What is the best way to learn these subtilities?

Also am I correct in assuming that choice "A" is wrong because we are comparing "growing fish" with their "natural growth rate.".?

"...suppliers are growing fish twice as fast as their natural growth rate.."

you'd look at one of the mainstays of gmat grammar - in fact, what is probably the single most important concept in sentence correction: PARALLELISM.
in choice c, there is clear parallelism (although there shouldn't be!) between 'growing fish twice as fast' and 'growing them naturally'. this indicates that the same agent, i.e. the supplier, is performing both of these actions.

parallelism is absolute: it encompasses grammatical parallelism as well as logical parallelism. therefore, if you see two clauses / phrases written in parallel, you can take their meanings to be logically parallel as well. ironically, the logical parallelism is why this particular sentence doesn't make any sense.

you are correct, if what you meant was that a faulty comparison (BAD PARALLELISM again) is being made.

because 'are growing fish' is a verb construction, it needs to be placed in parallel with another verb construction. choices a and b don't do that (there's no verb in 'their natural growth rate').