Genetic engineering sometimes entails plant or animal genes’ getting spliced into other species’ DNA, either to improve crop yields or for warding off insects or disease.


(A) plant or animal genes’ getting spliced into other species’ DNA, either to improve crop yields or for warding off

(B) plant or animal genes that get spliced into the DNA of other species, either for improving crop yields or the warding off of

(C) the splicing of plant or animal genes into other species’ DNA, to either improve crop yields or to ward off

(D) splicing plant or animal genes into the DNA of other species, either to improve crop yields or to ward off

(E) splicing of plant or animal genes into the DNA of other species, for either the improvement of crop yields or to ward off



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 off

If we use the infinitive form after “either” , we have to use infinitive form after “or”. Option A uses both infinitive- to improve and gerund- for warding off. 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 an 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. 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 off

Either 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. 


We can get to the answer by looking for the correct parallel structure.