Passage Summary: Scientists think the Amazon used to flow into the Pacific because it contains fish related to ancient Pacific species, even though mountains now block that path.

Conclusion: The Amazon River likely flowed into the Pacific Ocean in the past.

Reasoning: Current Amazon fish are descendants of extinct saltwater fish found in the Pacific but not the Atlantic, despite the river now being blocked from the Pacific by the Andes.

Analysis: The argument relies on a biological link to suggest a geological history. To strengthen this, we need more evidence that connects the Amazon's path to the Pacific or explains away the current mountain barrier. Look for an answer that provides geological proof, such as the Andes being younger than the fish species, or additional biological data that makes a Pacific connection more plausible than an Atlantic one.