I'm near ATL and taking August! Having a group for accountability would be great :)

The local resident is only talking about the relationship between small fish in the pond and algae. The resident never brings up a relationship between small fish and large fish which is why A is irrelevant.

D on the other hand talks specifically about the relationship between small fish and algae which is in the right direction.

Flaw questions do not ask you to add information (like maybe something about larger fish) to weaken the argument. Its asking you to describe what is already there that is weak, and there is no mention of larger fish or even a hint about a comparison between small and large fish.

Another way I like to think about it is diagramming out the argument structure.

Premise: algae and dead small fish (corr)

Conclusion: algae -c-> dead small fish (caus)

The whole argument and revolves around algae and dead small fish, and the author's reasoning is concluding causation from a correlation between algae and dead small fish. That's the biggest flaw in the resident's argument. A does not attack this line of reasoning while D does.

I hope this makes sense :)) I used to get caught up in the small details of a stimulus (which are important), but it is more important to understand the stimulus and its reasoning as a whole before delving into smaller details.