Logical reasoning PrepTest 143 · Section 3 · Question 17

Incorrect. Difficulty doesn't factor into what would work most effectively—it might only factor into an argument about practicality.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Strengthen with Necessary Premise

Stimulus Summary:
Rockets are most effective when pressure at the nozzle is equalized.
Low altitudes Ð Atmospheric pressure is high, and short nozzles are best
Thin upper atmosphere Ð Long nozzle is better
Conclusion Ð To work most effectively, all rockets must have both types of nozzles

Answer Anticipation:
Starting with the conclusion, we have a pretty strong statement—"all" rockets "must" have both types of nozzles. Why do they need both? Because one works better at low altitudes, whereas the other works better in the thin upper atmosphere. Well, in order to need both, then, the rocket would need to be traveling to both layers of the atmosphere. It's hard to imagine how they could avoid flying through low altitudes—after all, they start on Earth—but it's possible that not all rockets end up making it up to the thin upper atmosphere. In concluding that all rockets need both types of nozzles, the argument is assuming that all rockets make it up that high.

Answer Explanation:
If some rockets don't pass through the thin upper atmosphere, there's no reason for them to have long nozzles, invalidating the conclusion that "all" rockets "must" have long nozzles. This answer is thus necessary for the argument.

Key Takeaway:
Be very careful with the phrasing of answer choices in Strengthen with Necessary Premise questions, especially as you get to the harder questions in the section. The writers of the LSAT love to throw small term shifts in here and there to see if you're paying attention.

Incorrect. The argument is about how best to set up a rocket, and what type of nozzle is most effective at different altitudes, so a rocket with a short nozzle doesn't need to not be able to make it to high altitudes—just needs to be less effective than a long nozzle once it gets there.

Incorrect. The stimulus is about what it takes for rocket engines to be most effective, and this answer is about what it takes for rockets to be effective, so there are some shifts that invalidate this answer.

Incorrect. Be careful—the stimulus doesn't talk about any areas with low atmospheric pressure. While low altitude has high pressure, that doesn't mean that the thin upper atmosphere has low pressure—it might just be lower. This is a very subtle shift, but it invalidates this answer.