Passage Summary: Nitrogen builds up in animals more and more as they eat things higher up the food chain. Since old cave bear bones have the same high nitrogen levels as modern bears that eat meat, the author concludes those old bears were meat-eaters too.

Conclusion: Prehistoric European cave bears did not limit their diet strictly to plants.

Reasoning: Nitrogen levels increase as one moves up the food chain, and the nitrogen levels found in Ice Age cave bear bones match those of modern bears that eat meat.

Analysis: The argument relies on a comparison between ancient bears and modern bears. To strengthen this, we need to ensure that the 'heavy nitrogen' levels in the environment haven't changed in a way that would spoil the comparison. For instance, if Ice Age plants were naturally much higher in heavy nitrogen than modern plants, the cave bears could have reached those high levels just by eating plants. Look for an answer that eliminates this possibility, confirming that the high nitrogen levels in the bones could only have come from a meat-inclusive diet.