Adam’s dog, a golden retriever named Hans, can respond to over 150 different commands. Adam cites this fact as evidence for his claim that Hans can understand the English language.
Which of the following, if true, casts the most doubt on Adam’s claim that Hans can understand the English language ？（）

A. Each of the 150 commands to which Hans responds involves both a spoken word in English and a distinctive hand sign.

B. Hans does not respond to the same commands when spoken to him by Adam’s French and Italian friends in their own languages.

C. Scientists have demonstrated conclusively that canine vocal chords are incapable of replicating many of the sounds used in the English language.

D. Animal behaviorists have demonstrated that even very young dogs surpass both wolves and chimpanzees--animals that are thought to be more intelligent than dogs--in the ability to understand human nonverbal communication.

E. The golden retriever is widely considered to be less intelligent than the border collie, and no border collie has ever been shown to truly understand the English language.

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