The way the stimulus is written heavily implies that efficiency pairs up with usefulness and redundancy pairs up with reliability. But this is a must be true question, and those individual links are not explicitly established in the stimulus, which only says [efficiency and redundancy] as a pair can achieve [usefulness and reliability] as a pair.

At first glance, the word causes should also throw up a red flag, but we actually can find causation in the phrase “used…to achieve.”

Contradictory is a strong word, and it does tell us efficiency and redundancy cannot both be completely achieved. But that’s different from saying neither one can be achieved.

If you’re looking for support in the “if, then” statements, just note that the concept of complete redundancy appears in neither.

(C) would be right if you changed “useful” to “completely efficient.” Contradictory is a strong word, and it does tell us efficiency and redundancy cannot both be completely achieved.

But the jump from “cannot be completely efficient” to “cannot be useful” isn’t explicated in the stimulus. Not that this matters in a must be true question, but the jump also doesn’t make much sense in the real world. It says:

(C)’s Jump: If something isn’t completely efficient, it can’t be useful.

As for finding support in the “if, then” statements, just note that the concept of complete redundancy appears in neither.

(D) presents the logical inverse of the stimulus’ last sentence, which isn’t logically equivalent and doesn’t follow from the stimulus’ other claims. Here’s a formal logic comparison:

Stimulus: Auditory System Imperfect → /All Permutations Understandable

(D): /Auditory System Imperfect → All Permutations Understandable

(E) expresses the valid inference laid out in the analysis section, just in the contrapositive.

Con: Completely Efficient → Auditory System Perfect
(E): /Auditory System Perfect → /Completely Efficient