Passage Summary: Scientists found old air with lots of iron dust and very little CO2. They think the iron dust acted like plant food for algae, which then ate up all the CO2.

Conclusion: The high levels of ferrous material in the atmosphere caused the increase in algae populations during the last ice age.

Reasoning: Ice-age air samples show high ferrous material and low CO2; since algae consume CO2, the ferrous dust must have acted as a fertilizer for them.

Analysis: To weaken this causal claim, we need to find a reason why the iron dust didn't cause the algae growth or why the CO2 drop happened for another reason. Perhaps the algae population didn't actually increase, or maybe the iron dust arrived after the CO2 had already dropped. Look for an answer choice that breaks the link between the presence of ferrous material and the growth of the algae.