Whereas a ramjet generally cannot achieve high speeds without the init

By using the word “whereas”, the opening clause introduces a comparison between one clause and another. Because that portion of the sentence is not underlined, we must compare the fact that a ramjet cannot achieve high speeds without assistance to something else.

As currently written, the sentence compares “a ramjet cannot achieve high speeds” to “high speeds can be obtained by scramjets.” This compares the fact that ramjets cannot do something with the fact that high speeds can be attained by something else. Although it may not be 100% wrong, that comparison is definitely not as good as comparing what a ramjet cannot do to what a scramjet can do.

Also, whatever immediately follows the parallelism trigger “and” should be parallel to something earlier in the sentence. In this case, the word “letting” is not parallel to anything earlier in the sentence. Logically, it should be parallel to “reduce”, but “reduce” is not parallel to “letting.” Instead, “reducing” would be parallel with “letting.”

But because (A) contains a less-than-ideal comparison and the parallelism is off, we can eliminate this answer choice.


(B) compares “a ramjet cannot achieve high speeds” to “that high speeds can be attained by scramjets.” As in (A), this comparison between the fact that a ramjet cannot do something and the fact that high speeds can be attained by something else is problematic.

Again, it would make sense to compare what a ramjet cannot do to a what a scramjet can do. It would even make sense to compare how high speeds cannot be attained to how high speeds can be attained. But (B) confuses the two. Eliminate (B).


(C) makes a mistake similar to the one found in (B). It compares that “a ramjet cannot attain high speeds” to “the ability of scramjets...is because....” It definitely does not make sense to compare what a ramjet cannot do to WHY scramjet’s have a certain ability. Moreover, as in (A), “letting” is not parallel to anything earlier in the sentence.

In addition, “the ability... is because they reduce...” simply does not make much sense. Perhaps, scramjets are able because they reduce..., but scramjets’ ability is not “because” anything. Eliminate (C).


(D) fixes the comparison issues found in (A), (B), and (C). A ramjet is properly compared to scramjets. At first glance, it also looks like (D) correctly parallels “letting” to something found earlier in the sentence (“reducing”), and (D) does not seem to contain any definite errors.

Now let’s think about meaning. (D) says that “scramjets...have the ability of attaining high speeds when reducing...and letting....” The word “when” explains AT WHAT TIME scramjets have the ability to attain high speeds. In other words, when scramjets reduce airflow compression, they can go fast. But what about when they do NOT reduce airflow compression? The author intends to explain HOW scramjets attain high speeds, not WHEN they do so.

There’s also some ambiguity in the parallelism. It seems like the author wants to say that scramjets have the ability of attaining high speeds when:

(1) REDUCING airflow compression at the entrance of the engine and
(2) LETTING air pass through at supersonic speeds.

But it could also be read as if scramjets have the ability of:

(1) ATTAINING high speeds when reducing airflow compression at the entrance of the engine and
(2) LETTING air pass through at supersonic speeds.

The fact that we have multiple ways to interpret the parallelism isn't ideal (especially since one of those ways does not seem to fit with the author's intentions). This is another vote against (D).

Finally, there are a couple issues with the phrase “have the ability.” First, idiomatically, scramjets “can attain high speeds” is simply more direct and clear than scramjets “have the ability of attaining high speeds.” But when we consider the literal meaning of the sentence, “able” implies agency, or the ability to act on one’s own behalf. And a scramjet, as an inanimate object, does not really possess agency and does not act on its own accord. So, a scramjet “can...”, but a scramjet does not really “have an ability.”

For all those reasons, we can eliminate (D).


Like (D), (E) corrects the comparison and parallelism issues found earlier in the sentence and contains no definite errors. The word “by,” as opposed to “when,” clearly shows that the author is explaining HOW scramjets attain high speeds. Also, by using “attain” rather than “attaining,” it is clear that “letting” is parallel to “reducing.”

(E) clears up the definite errors and meaning issues found in the previous choices, so it is the best answer choice.