hmm.
there's an upside and a downside to understanding the exact nature of grammatical parallelism.
the upside: MOST sentences involving parallelism will contain
exact parallelism. in other words, the parallel structures will have
exactly the same grammar and/or form.
in these cases, you could approach the sentences with a completely mechanical viewpoint -- to the extent that you could still analyze them, even if you had no idea what any of the words meant -- and still get the correct answers.
the downside: there are some official sentences (and, of course, sentences in real writing) in which the parallelism is
not completely exact. these are sentences in which you can't use exact parallelism without distorting or destroying some essential semblance of the meaning.
for an example,
click here.
this is another one of these sentences. choice (d) appears to have better parallelism -- and, from a completely mechanical perspective that is devoid of consideration for what the sentence is saying, it does -- but the problem is that this choice loses an essential component of what it is supposed to say.
in choice (e), the infinitive "to acquire" imbues the sentence with the meaning of PURPOSE: i.e., the breeders are using crossbreeding
with the specific purpose of acquiring these characteristics. choice (d) simply doesn't say this; the word "because" tells us that there must be
some sort of causal connection, but it's unclear exactly what that connection is.
conversely, it's also impossible to structure the second part of the sentence as a straight infinitive -- i.e., you can't just say "to provide hybrid vigor" -- because then we would lose the idea that this is only speculation.
admittedly, you probably won't see much of this. for every one sentence in which you have this sort of imperfect parallelism, there will probably be 10 to 20 sentences in which the parallelism is absolutely 100% grammatically perfect.
therefore -- especially if you are not a native speaker of english -- you may want to accept the (extremely infrequent) presence of these sorts of sentences as a sort of collateral damage. in a problem like this one, it's pretty much going to be impossible for you to know when to pick the imperfect parallelism over the perfect parallelism, unless you have a grasp of the language that is just as exquisite as that possessed by native speakers.