Passage Summary: Roberta only acts a certain way when she's sleepy. Since she's sleepy and doing one of those things, the author assumes she must be doing the other thing too.

Conclusion: Roberta is almost certainly irritable right now.

Reasoning: Roberta only gets irritable or loses things when she's tired, and currently, she is tired (yawning) and has just lost her keys.

Analysis: This argument suffers from a classic formal logic error regarding necessary conditions. While being tired is a necessary condition for Roberta being irritable, it isn't a sufficient one. Just because she is tired and lost her keys doesn't mean every other 'tired-only' behavior must also be occurring. Look for an answer that points out that the author treats a condition necessary for a state as if it guarantees that state.