Sentence Correction

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
(B) with such rapidity that the population was one-half
(C) so rapidly the population divided in half
(D) so rapidly that the population was halved
(E) in such rapidity that the population is halved

Confused between D and E again.. what's the OA?

So X that Y.

IMO D.

The 2/3 split is "rapidity" (A, B, and E) vs "so rapidly" (C and D) "So rapidly" is more direct, so we'll eliminate A, B, and E.

C says the population "divided in half"; by using active voice, it makes it sound like the population divided itself.

D uses the passive voice "was halved," which makes it clear that something else is doing the halving.

STRAIGHT D

IMO C
Why ? Because the verb tense was vs in is clearly sited with the indicator - "In the past twenty years"

-> Past tense of in the last twenty years !
-> 1960 to 1980, the verb tense indicate is pointered at 1980.
-> GMAT prefers active voice - ALWAYS !

@Bill, Awesome reply ! - Kudos

HTH!
Penguinfoot