Passage Summary: Ships use water tanks for stability, but this water often carries critters to new places where they cause trouble. The author suggests emptying and refilling these tanks in the middle of the ocean to kill off any hitchhikers.

Conclusion: Exchanging ballast water in the middle of the ocean is a practical solution for preventing the spread of invasive sea creatures.

Reasoning: Coastal and midocean creatures typically cannot survive in each other's environments, so swapping the water mid-journey should neutralize the threat.

Analysis: The argument relies on a critical missing link: that the midocean creatures themselves won't become the new invasive species. For this plan to be 'viable,' we must assume that the new batch of midocean water doesn't contain organisms that could survive and wreak havoc in the next coastal port. Look for an answer that ensures the cure isn't just as bad as the disease. We also need to assume that the act of swapping water doesn't compromise the ship's stability during the process.