Logical reasoning PrepTest 121 · Section 1 · Question 25

Incorrect. This answer might establish why the microbe species can't be grown in isolation, but it doesn't address whether or not that prevents complete knowledge of these species from being attained. Additionally, the reproduction of this system is never mentioned in the stimulus, and so it's likely out of scope.

Incorrect. As with (A), the reproduction of the complex system is never mentioned in the argument and is thus likely out of scope. And also like (A), this answer doesn't address the new concept in conclusion—the impossibility of attaining complete knowledge.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Strengthen with Sufficient Premise

Stimulus Summary:
Microbe species are interdependent, so microbiologists can't cultivate a single species alone, and thus they don't have complete knowledge of most microbe species.

Answer Anticipation:
The science here is meant to distract you from the relatively straightforward logic of this question. If we strip all the science out, the argument boils down to:
Microbiologists can't cultivate most microbe species alone (i.e., without other microbe species), so microbiologists don't have complete knowledge of most microbe species.

This question type frequently introduces new terms in the conclusion that need to be connected to an idea in the premise, and that plays out here. The conclusion talks about "complete knowledge," but that's missing from the premises. What does the author assume guarantees that microbiologists lack complete knowledge of most microbe species? That they can't be grown in isolation. So the correct answer should connect these ideas:
If a species can't be grown in isolation, then complete knowledge can't be gained for it

Or, in other words (the contrapositive):
In order to gain complete knowledge of a species, scientists must be able to grow it in isolation

Answer Explanation:
If we treat "unless" as "if not," we can diagram this answer as: not Cultivate species in isolation → not Complete knowledge of a species of microorganism. The stimulus establishes that these microbe species can't be cultivated in isolation, and so this answer guarantees that complete knowledge of these microbes can't be attained. That's what the conclusion states, so this is the correct answer.

Key Takeaway:
Always be on the lookout for new concepts in the conclusion of Strengthen with Sufficient Premise questions. When one shows up, it needs to be connected to something from the premises in the correct answer.

Incorrect. Since the conclusion is about all microbiologists, this answer about "at least some" of them is almost certainly too weak to justify it. This answer is more of a necessary assumption, not a sufficient one.

Incorrect. This answer establishes that it will always be impossible to cultivate these species in isolation, but it was already established that that is currently the case, and the conclusion is about what is true now, so this answer doesn't add anything relevant to the argument. Additionally, it doesn't talk about the new concept in the conclusion—the complete knowledge.