Even though apes are the only nonhuman creatures able to learn human language, no ape has ever used its human language skills to ask such philosophical questions as, "How am I different from all other creatures? " Ttherefore, philosophical thought is unique to humans. The conclusion in the passage above relies on which one of the following assumptions? ['Apes are incapable of thinking in human language.', 'Human language is unique to humans.', 'Speaking in human language is easier than thinking in human language.', 'Philosophical thought can be expressed only in human language.']