Passage Summary: Musicians have bigger brain parts for hearing piano sounds than non-musicians do, so the argument says playing the piano must be what grew those brain parts.

Conclusion: Practicing an instrument causes changes in the brain's physical structure.

Reasoning: Skilled musicians have a larger specific brain area compared to people who don't play.

Analysis: This is a classic correlation-versus-causation error. The author assumes that the activity (playing piano) caused the brain structure, but it is just as possible that people born with larger brain areas in that region are simply more likely to become skilled musicians. We cannot determine the direction of the relationship from the data provided. Look for an answer that points out this 'chicken or the egg' problem or suggests the possibility of a pre-existing condition.