What bothers me about this explanation is that he says "what does fish have to do with the premise", which seems contradictory to explanations of solutions to other weakening questions. We are explicitly taught to not attack the premise or the conclusion, but rather, the support. Answer choice A does not attack the premise or the conclusion. Plus, if there is a word for fish, it does CAST a doubt on whether or not the Proto-Indo-Europeans lived near oceans/seas. It doesn't destroy the argument, but it could weaken it.

In other answer choices for weakening questions we have to make bold assumptions all the time, but this time it's wrong to assume that fish come from the sea/ocean?

I am not refuting B as the correct answer choice but simply stating that the explanation to why A is wrong is somewhat contradictory to previous answer explanations of the same question type.

send me the link!!

I too am in the same boat. And I'm unsure on how to fix it.