25. This LSAT assumption question works this way:
| P1: Microbe species live interdependently. | |
| Secondary Conclusion: Impossible to cultivate single species in isolation. | |
| _______________________ | |
| C: Microbiologists lack complete knowledge of >50% of species. |
The author assumes that complete knowledge of a species requires the ability to cultivate that species in isolation, and the correct choice, (C), works this way:
| Complete knowledge g Can cultivate species in isolation. |
To negate (C), change "No" to "A", "unless" to "even if", and place "not" before cultivate. This negation weakens the argument and thus (C) is a required assumption.
(A) is irrelevant. The argument is based on cultivation of a single species separated from the community, not the entire community. Negate (A) by stating "not impossible". The argument is unaffected.
(B) also refers to the cultivation of the entire community, which is irrelevant.
(D) alters the conclusion by including "at least some" whereas the argument implies "all", and (D) contains "any" whereas the argument states "most".
(E) would make it impossible for the microbiologists to gain complete knowledge, but the author's argument is based on whether microbiologists currently have complete knowledge, not whether such knowledge is ever obtainable. Negate (E) by changing "No" to "at least one". The argument is unaffected.
26. Reza provides a stack of conditional statements:
| Language g Use of verbal signs for objects | |
| Language g Use of verbal signs for feelings | |
| Use verbal signs for objects g Human | |
| Use verbal signs for objects g Ability to distinguish between objects | |
| Ability to distinguish between objects g Conceptual thought |
(A) is the correct choice, as seen by connecting statements:
| Language g Use verbal signs for objects g Ability to distinguish between objects g Conceptual thought |
(B) is incorrect because conceptual thought is mentioned last, and requires nothing (that we know of).
(C) is a guess. Just because the passage does not connect feelings and conceptual thought, this does not imply anything about the relationship between the two.
(D) could be true, but could be false. We know that the use of verbal signs requires that one be human and be able to distinguish between objects. We also know that to be able to distinguish between objects requires conceptual thought. Thus, we know that all those who use verbal signs for objects are capable of conceptual thought. But, it's possible that one is human and does not use verbal signs for objects. It's also possible that one is human but cannot distinguish between objects. Finally, it's possible that one is human and incapable of conceptual thought.
For (E), we do know the contrapositive of the inference drawn in (A):
But (E) states: