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Question 127·200 Super-Hard SAT Reading Questions·Craft and Structure

Some animal-communication researchers have argued that calls made by Japanese tits work in a syntax-like way: listeners respond not only to the individual calls in a sequence but also to their order. A simpler account is that the birds react only to overall arousal, since more urgent situations may produce more intense calls. In a playback experiment, Keiko Suzuki's team independently varied call order and acoustic intensity. If the arousal account by itself were sufficient, swapping the order of two calls while preserving their intensity should leave the listeners' response unchanged. Instead, the birds approached the speaker only when the calls occurred in the naturally observed order, even when the reversed sequence was equally intense. The result does not establish that the calls have all the properties of human syntax, but it does undermine an explanation based solely on arousal.

Which choice best describes the function of the underlined sentence in the text as a whole?