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Some ores deep in the Earth are rich in iron particles. These particles are too vulnerable to the corrosive effects of oxygen to have accumulated under the oxygenated atmosphere of today. Therefore, when the ores were deposited millions of years ago, the atmosphere must have contained less oxygen than it does today.
The argument above requires which of the following assumptions?
| Text from Passage | Analysis |
| Some ores deep in the Earth are rich in iron particles. |
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| These particles are too vulnerable to the corrosive effects of oxygen to have accumulated under the oxygenated atmosphere of today. |
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| Therefore, when the ores were deposited millions of years ago, the atmosphere must have contained less oxygen than it does today. |
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The argument starts with an observation about iron-rich ores, then explains why these ores couldn't form in today's atmosphere, and concludes that the ancient atmosphere must have been different (less oxygenated) to allow these ores to form.
When the ores were deposited millions of years ago, the atmosphere contained less oxygen than it does today.
This is a cause-and-effect argument working backwards. We observe an effect (iron-rich ores exist) and know a constraint (they can't form in high-oxygen environments), so we conclude the cause must have been different conditions (lower oxygen levels in the past).
Assumption - We need to find what the author must believe to be true for their conclusion to hold. This is something that, if false, would make the argument fall apart.
The argument makes specific claims about iron particles being 'too vulnerable' to current oxygen levels and concludes the ancient atmosphere 'must have contained less oxygen.' These are precise comparative claims about vulnerability thresholds and atmospheric composition across time periods.
For assumption questions, we identify ways the conclusion could be falsified while respecting the given facts. The author concludes that ancient atmosphere had less oxygen because iron particles exist today but are too vulnerable to current oxygen levels. We need to find what must be true to bridge the gap between 'iron particles exist in ores' and 'therefore ancient atmosphere had less oxygen.'
This states that iron particles were exposed to the atmosphere when the ores were deposited. This is absolutely critical for the argument to work. The author concludes that ancient atmosphere had less oxygen based on the fact that iron particles survive in ores but are too vulnerable to today's oxygen levels. But if the iron particles were never exposed to the atmosphere during ore formation (perhaps they formed in sealed underground environments), then their survival tells us nothing about atmospheric conditions. Without this assumption, the entire argument collapses. This is the correct answer.
This claims that ores were more vulnerable to corrosion in the past than today. This actually contradicts the argument's logic. The author argues that iron particles survived in the past because there was less oxygen then, not because they were more vulnerable. If anything, the argument suggests conditions were less corrosive in the past due to lower oxygen levels. This reverses the argument's reasoning.
This states that ores are deposited at various depths. This is completely irrelevant to the argument about atmospheric oxygen levels over time. The argument focuses on when the ores formed (millions of years ago vs. today) and atmospheric conditions during formation, not where they're located vertically in the Earth. Depth variation doesn't impact the reasoning about historical atmospheric composition.
This says oxygen levels remained constant during the ore deposition period. This isn't necessary for the argument. The author only needs the ancient atmosphere to have had less oxygen than today's atmosphere. Whether oxygen levels fluctuated during that ancient period doesn't affect the core comparison between ancient and modern atmospheric conditions.
This compares iron content between surface and deep ores. This is irrelevant to the temporal argument about atmospheric changes over millions of years. The argument is about comparing ancient atmospheric conditions to modern ones, not about comparing different locations in the Earth today. The spatial distribution of iron particles doesn't impact the historical reasoning.