Excellent questions. I will try to model how I approached this question, then answer yours if they haven't been addressed.
In my initial reading of the stimulus, I noticed that they were setting up a sort of paradox (not unlike the resolve the paradox questions!). On one hand, we know that the Andes cut off the Amazon from the Pacific. On the other hand, we know that fish in the Amazon descended from fish from the Pacific. It is important to notice the word "yet" as it sets up the paradox. Now, after reading the stimulus, I am expecting a resolve the paradox or a causal strengthen or weaken.
Why causal? Well, one could interpret the stimulus as "pacific fish ancestors" --> "Amazon fish".
Now, we can see that the question stem is a strengthen. So, were are looking for something that supports the causal link. In the stimulus, what is the statement about the Andes doing? Well, it is a counter-premise, since it seems to be evidence against the idea that Pacific fish were ancestors of Amazonian fish. The fish couldn't have walked over the Andes, right?
Now, we can prephrase. What if the Andes weren't there? Then the Pacific fish could have freely traveled inland to the Amazon basin. And we have words that support this idea.
The land through which the Amazon flows is now cut off from the Pacific Ocean to the west by the Andes Mountains.
Why would anyone put the word "now" in that sentence? The only reason is to guide our prephrase.
So, now I would do a key-word prephrase and look in the answers for something about the Andes mountains not always having been there. Only answer choices (B) and (D) contain the word Andes so we take a closer look. Only (B) talks about the period before the Andes formed. And that is our answer. It is a good example of how prephrasing can save you time by not reading all 5 answers thoroughly because you have a strong idea of what the correct answer will contain.
So, all answer choice (B) tells us is that the ancestor species existed
prior to the formation of the Andes. And that is what we prephrased and that is what we need. Why? Well, that removes our counterpremise. Since the species existed before the formation of the Andes, they could have migrated from the Pacific to the Amazon basin without that obstacle. Therefore, they could have evolved eventually to today's Amazon fish.
I think you got taken off track by focusing on the "now-extinct" part of the answer choice. That is not new information but was contained in the stimulus. We should not infer anything about how they went extinct or when.
And how does either of those show that the Amazon river once flowed into the Pacific ocean?
(B) tells us two things: the Andes weren't always a barrier between the Pacific Ocean and the Amazon and the ancestor fish existed during this period. We need both of these things to be true for the scientists to be correct. If the Andes were always a barrier, then there is no way the fish could get to the Amazon from the Pacific. If the fish did not exist in a time period where the Andes were not a barrier, then they also could not have been ancestors. But we know that the ancestor-fish could have gotten to the Pacific. Or at least the main reason against this hypothesis, namely, the existence of the Andes mountains, wasn't always a reason against, thus it strengthens (though doesn't prove) the scientists hypothesis. And that's all we need to do for this question.
