Passage Summary: A linguist claims that because a specific sentence meets the requirement of being diagrammable, it must be a valid sentence that people will recognize as correct.

Conclusion: Speaker X's sentence will be recognized as grammatical by speakers of its language.

Reasoning: Being diagrammable is a necessary condition for a sentence to be grammatical, and all grammatical sentences are recognized by speakers; Speaker X's sentence can be diagrammed.

Analysis: The linguist is making a classic logical error by confusing a necessary condition with a sufficient one. Just because a sentence *must* be diagrammable to be grammatical doesn't mean that *any* sentence that can be diagrammed is automatically grammatical. To spot the flaw, look for an answer that highlights the possibility of a sentence being diagrammable yet still failing to be grammatical for other reasons. It is like saying you must have a ticket to board a plane; having a ticket doesn't mean you are actually on the plane yet.