We can get to the answer by looking for parallel structure.
(A) plant or animal genes’ getting spliced into other species’ DNA,
either to improve crop yields or for warding offIf we use the infinitive form after “either”, we have to use the infinitive form after “or”. Option A uses both infinitive- "to improve" and gerund- "for warding off" . Option A breaks parallel structure.
Eliminate.
(B)
plant or animal genes that get spliced into the DNA of other species, either
for improving crop yields or the warding off of
The meaning of the sentence is that
genetic engineering sometimes entails splicing plant or animal genes into other species’ DNA…either
to improve…
or to ward off..
B says Genetic engineering sometimes entails plant or animal genes either for improving crop yields or the warding off. This is illogical.
either for improving or the warding off- not parallel. It should have been- either for improving or for warding off.
However, the use of infinitive is preferred as we are talking about intent here.
Eliminate.
(C) the splicing of plant or animal genes into other species’ DNA, to either improve crop yields or to ward off
If “to” precedes “either”, it is common to both the verbs- to either improve or ward off.
Option C repeats “to” after “or”.
Eliminate.
(D)
splicing plant or animal genes into the DNA of other species,
either to improve crop yields or to ward offEither to improve or to ward off.
Correct.
(E) splicing of plant or animal genes into the DNA of other species, for either the improvement of crop yields or to ward off
Breaks parallel structure.
Eliminate.
Vishnupriya
GMAT Verbal SME
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Crackverbal Prep Team
www.crackverbal.com