Passage Summary: Researchers noticed babies use their mouths differently when babbling than people do when making random noises, leading them to conclude babbling is actually a form of speech.

Conclusion: Babbling is a linguistic activity rather than just random noise.

Reasoning: Babies open the right side of their mouths wider when babbling, whereas previous studies show people open the left side wider for non-linguistic sounds.

Analysis: The scientist's argument proceeds by using a comparative method to categorize a behavior. First, a distinction is established: non-linguistic sounds are associated with a specific physical trait (left-side mouth dominance). Then, the scientist shows that babbling lacks this trait and instead exhibits the opposite (right-side dominance). By showing that babbling does not fit the profile of 'non-linguistic' vocalization, the scientist concludes it must be 'linguistic.' Focus on identifying the description that highlights this use of contrasting physical evidence to determine the nature of an activity.