Identify the Question Stem: The word if true and strengthen indicate that this is a Strengthen the Argument question.

Deconstruct the Argument: The author provides some specific evidence of higher cognitive abilities of parrots (such as a large vocabulary) and details how one parrot, Alex, would respond aggressively when people didn't respond correctly to what he'd asked. The author concludes from this evidence that humans and primates are not the only animals capable of using language to communicate.

Pause and State the Goal: On Strengthen questions, the goal is to find a new piece of information that, if added to the argument, would make the conclusion at least somewhat more likely to be valid. In this case, what would make it more likely to be true that animals other than humans and primates are capable of using language to communicate?

Work from Wrong to Right:
(A) The argument does mention that dolphins may exhibit higher-level cognitive functions, but this choice does not address the language claim made in the argument. The dolphins could be trained in a way that does not involve any language cues.
(B) Hand signals are a type of language communication, but gorillas are primates (as noted in the first sentence of the argument). The argument is making the case that other animals, not just primates, can communicate using language; this choice doesn't help to further that argument.
(C) The argument does mention that dogs may exhibit higher-level cognitive functions, but this choice does not address the language claim made in the argument. Sensing moods and showing concern are not the same as using language to communicate.
(D) Chimpanzees are primates, as noted in the first sentence of the argument. The argument is making the case that other animals, not just primates, can communicate using language; this choice doesn't help to further that argument. (Also, memorizing a sequence of keys is not the same thing as using language to communicate.)
(E) CORRECT. This is a tricky correct answer! The argument stated that Alex would react aggressively when people didn't respond to him in the way he wanted—but perhaps he acted aggressively all the time. Or perhaps the aggressive behavior came out randomly and wasn't actually tied to not getting what he wanted. But if Alex both acted aggressively when people didn't respond the way he wanted and reacted normally when people did respond the way he wanted, then it's at least a little more likely that he did know what he was saying—that is, that he was actually using language to communicate.

The correct answer is (E).

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