The number of mountain gorillas is declining with such rapidity that

I had marked A , I am not really sure why D is correct
The tense used in the sentence is active and present where as the correct option D is of past tense..

The number of mountain gorillas is declining with such rapidity that the population is one-half in the twenty years between a count made by George Schaller in 1960 and the one made by Dian Fossey in 1980.

(A) with such rapidity that the population is one-half- use of present tense "is"-incorrect.2. use of with such rapidity to describe the verb "is declining" incorrect. It should be an adverb.
(B) with such rapidity that the population was one-half- 1.Same aprep error as A. 2."The population was one half.." wrong usage.
(C) so rapidly the population divided in half- wrong meaning
(D) so rapidly that the population was halved- correct
(E) in such rapidity that the population is halved- same prepostion error as A. 2. Use of present tense is incorrect as the action took place in the past.

Sol Courtesy: expertsglobal

“with such rapidity” and “in such rapidity” are unidiomatic and hence, A, B, and E are incorrect.
C wrongly suggests that the “population divided in half”. Furthermore, “that” is missing before “population”.
In D, “so rapidly…” is idiomatic through “...so + adjective/adverb + that..” structure. “was halved” implies that the population has already been halved by outside forces such as poachers, diseases, etc. Hence, D is the best choice.
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i thought because of "mountain gorillas is declining" so population "halved". since population halved is a result then it must be in present tense that's why i did not choose D. what's wrong with my reasoning???

Yes, according to the statement it 'is' declining but the population 'halved' in a period (1960-1980) in the past and therefore 'was' is apt.

Agreed!

For example, when we say, "The glaciers are melting," we are describing an ongoing action--one that started in the past and that will continue (for an unknown amount of time) into the future. Has the melting been going on for a few days? Weeks? Years? Decades? Centuries? How long will this continue? We don't know based on the verb tense, and we have to rely on any contextual clues.

Similarly, the verb "is declining" represents an ongoing action -- one that is occurring now and that will continue into the future. Based on the context, we know that this action has being going on since at least 1960, that the action is still going on today, and that the action will continue for an unspecified/unknown amount of time.

On the other hand, the verb "was halved" refers to a specific time in the past (1960-1980), a subset of the time frame of the verb "is declining."
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Here's the official explanation provided by the GMAC for this question:

According to the sentence, the number of mountain gorillas is declining so quickly that the population in 1980 was half that in 1960. The verb form is declining indicates that the decline continues at the present time. However, the decline in population between 1960 and 1980 is reported as having already occurred, so the verb in the highlighted portion of the sentence should be in a past tense. To convey the intended meaning, the sentence must clearly compare the population in 1960 with the population in 1980. The wording should be concise.

Option A: The phrase with such rapidity is an unnecessarily wordy substitute for so rapidly. The present tense is does not convey the intended past-tense meaning. The phrase the population is one-half in the twenty years is incoherent; it does not say what the population is one-half of, and thus does not convey the intended sense of the population decline.

Option B: The phrase with such rapidity is an unnecessarily wordy substitute for so rapidly. The phrase the population was one-half in the twenty years is incoherent; it does not say when the population was one-half of what, and thus does not convey the intended sense of the population decline.

Option C: The phrase the population divided in half confusingly suggests that the population of mountain gorillas separated into two groups, whereas the sentence's more likely intended meaning is that the number of mountain gorillas declined. The phrase so rapidly the population divided could be misread as meaning so the population rapidly divided.

Option D: Correct. This concise wording clearly conveys the intended sense of the population declining between 1960 and 1980. The past tense was is appropriate to the meaning. The use of that clarifies that rapidly modifies declining rather than was; so rapidly …that introduces a clause indicating a consequence of the rapidity.

Option E: The phrase in such rapidity is not only unnecessarily wordy but also idiomatically incorrect; the correct preposition would be with, not in. The present tense is conveys neither the past-tense meaning of the clause nor the sense of a decline over time.

The correct answer is D.

Please note that I'm not the author of this explanation. I'm just posting it here since I believe it can help the community.

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