challenge LSAT 4-21

Snazzy

21.    Efficiency and redundancy are contradictory characteristics of linguistic systems: however, they can be used together to achieve usefulness and reliability in communication. If a spoken language is completely efficient, then every possible permutation of its basic language sounds can be an understandable word. However, if the human auditory system is an imperfect receptor of sounds, then it is not true that every possible permutation of a spoken language’s basic language sounds can be an understandable word.
If all of the statements above are true, which one of the following must also be true?
(A) Efficiency causes a spoken language to be useful and redundancy causes it to be reliable.
(B) Neither efficiency nor redundancy can be completely achieved in spoken language.
(C) If a spoken language were completely redundant, then it could not be useful.
(D) If the human auditory system were a perfect receptor of sounds, then every permutation of language sounds would be an understandable word.
(E) If the human auditory system is an imperfect receptor of sounds, then a spoken language cannot be completely efficient.

As we all know, when 'if X is not true, then B is not true" works, then "if B is true, then X is true".  But E, the claimed answer, don't respect this rule at all. What the hell is that?

mindfree

I do not see any problem with the answer. The reasoning in the question is:

complete efficiency (A)--> every permutation is understandable (B); auditory system is imperfect (C) --> every permutation cannot be understandable (neg B)

From the above reasoning, I can draw a conclusion that: C --> neg A.