Passage Summary: Since a rock from Mars looks like Earth rocks from before life started, there probably wasn't any life on Mars.

Conclusion: It is improbable that life ever existed on the planet Mars.

Reasoning: A Martian meteorite shows isotope ratios that, on Earth, are only associated with the period before biological activity started.

Analysis: This argument assumes that life on Mars would affect isotopes in the exact same way life on Earth does. To weaken this, we could suggest that Martian life uses different biological processes or that the meteorite isn't representative of the whole planet's history. Since this is a 'Weaken Except' question, the correct answer will likely be irrelevant to the conclusion or perhaps even slightly support it. Focus on identifying the four choices that create doubt about using Earth's timeline as a template for Mars.