Passage Summary: Some fish in lakes lack the armor their ocean relatives have; the author thinks this is because being big is a better way to stay safe in a lake than being armored.

Conclusion: For lake sticklebacks, having a larger body size provides a more effective defense against predators than having protective armor.

Reasoning: Ocean sticklebacks have armor while lake sticklebacks do not; because armor is known to slow down a fish's growth, the lack of armor in lake fish suggests they are prioritizing growth speed for defense.

Analysis: The author makes a leap from a physical trade-off (armor vs. growth speed) to a defensive strategy (size is better than armor). To weaken this, look for an answer that provides an alternative reason for the lack of armor that has nothing to do with size-based defense. For instance, perhaps the lake simply lacks the calcium necessary to build armor, or the specific predators in the lake are not deterred by armor anyway. We need to find a reason why the 'size as defense' explanation isn't the only—or even the best—conclusion to draw from the lack of plating.