Passage Summary: A person claims they can prove a thermometer is accurate by looking at plants: if the rhododendrons aren't curled (meaning it's at least 0°) and the crocuses aren't open (meaning it's at most 2°), then the temperature is definitely right around 1°.

Conclusion: A thermometer reading of 1°C is confirmed as accurate within one degree if one observes uncurled rhododendron leaves and closed crocus blossoms simultaneously.

Reasoning: Rhododendrons curl below 0°C and crocuses open above 2°C, so if the leaves are uncurled and the flowers are closed, the temperature must be between 0°C and 2°C.

Analysis: To weaken this argument, we need to find a reason why the plants might not be perfectly synchronized with the air temperature shown on the thermometer. The author assumes the plants react instantly and exclusively to the current air temperature. If there is a significant time delay in how these plants react, or if they respond to different factors like soil temperature or sunlight, the 0–2°C window might be invalid. Look for an answer that introduces a variable that separates the biological reaction of the plants from the immediate air temperature.