Passage Summary: The Hydro is a hit while similar fuel-efficient cars are flopping. Since it's not actually cheaper or more efficient than the others, the author figures people must be buying it just to show off how 'green' they are.

Conclusion: The success of the Hydro is likely caused by consumers' desire to appear environmentally conscious to others.

Reasoning: The Hydro is selling better than its competitors despite having similar prices and fuel efficiency, which rules out the manufacturer's explanation for its success.

Analysis: The author uses a process of elimination to reach a causal conclusion, but this only works if all other possible causes have been ruled out. The argument 'needs' to assume that there isn't some other practical or aesthetic difference—like a better warranty, superior safety ratings, or a more attractive design—that explains why the Hydro is outperforming its rivals. Look for an answer that eliminates one of these alternative explanations.