Logical reasoning PrepTest 143 · Section 4 · Question 20

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Strengthen with Necessary Premise

Stimulus Summary:
Some important medicines were discovered in plants only found in rain forests. Thousands of undiscovered plants exist there; it's likely some have medicines in them. Therefore, if rain forests disappear, some medicines will never be developed.

Answer Anticipation:
The conclusion here is that some medicines "will never" be developed, based on the prior discovery of important medicinal substances in rain forest plants. Generally, making a prediction about what will happen in the future based on the past is a flawed way to argue because something could have changed.

What has changed here? Well, some important medicines have already been developed based on the substances in these plants. Since the conclusion is about new medications being discovered, the argument is flawed because it ignores a change from the past—that we've already discovered some medicines, so maybe we've discovered all the new medicines the rain forest plants have to offer. For this argument to work, there has to be some substances out there unique to plants in the rainforest that have yet to be found.

Answer Explanation:
If we negate this answer—if there aren't substances left to be discovered that are different from those already discovered—then we wouldn't lose any medicinal substances if the rainforests disappeared. When negated, this answer destroys the argument, so it's the correct answer.

Key Takeaway:
While the conclusion has a rather extreme word in it related to the timeline ("never"), look back at the other statements—whenever it's talking about medicines or plants, it uses weak language. "Many" medicines; "many such plants"; "important types of medicine"—all of these are akin to some statements. Now, look at the answer choices—"Most" plants; "majority of plant species"; "any substance of medicinal value"—these are all much stronger. In general, answers that are stronger than the argument exceed what is necessary for the argument to work. Use this rule of thumb when analyzing Strengthen with Necessary Premise questions—it can be a great way to make some eliminations!

Incorrect. If anything, this answer hurts the argument, which is a fatal feature of an answer in a Strengthen with Necessary Premise question.

Incorrect. This answer is too strong. It's not necessary that most of these plants have substances of medicinal value, just that some of them have unique substances of medicinal value.

Incorrect. This answer can seem tempting—after all, if scientists wouldn't have discovered these substances even if studying the plants, then those plants disappearing didn't prevent something that would have happened. However, as with many Strengthen with Necessary Premise questions, this answer is too strong. The argument only requires that scientists would discover any unique substances, not all substances. If they'd miss a few that had already been discovered, the argument could still hold together.

Incorrect. The conclusion isn't about what should be done—it's just about the impact of tropical forests not being preserved. As such, anything about what "should" be done is out of scope.